Thinly sliced onions and chunky mushrooms caramelize with butter and Marsala wine to make the perfect jammy topping for this easy crostini appetizer with a silky sweet taste.
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This recipe is brought to you by Vermont Creamery
Carmelized onions and mushrooms are my jam. Literally. Jammy. That’s what happens when the sugars of these two basic ingredients hit the heat and do their dance with sweet cultured butter. The sugars bloom and the onion and mushroom’s earthy flavors deepen to create a savory jam that becomes nearly as sweet as any preserve you’ll spread on toast.
That’s what makes this mixture the perfect topping for hand-held toasted crostini appetizers. And they are super easy to prep and cook too. Let’s party!
What’s in Onion and Mushroom Crostini
Crostini are one of the most versatile appetizers you’ll find on the party table—and one of the most popular. Known in Italian as “little toasts”, crostini are thin slices of toasted bread topped with a wide array of savory toppings.
This appetizer recipe combines two of my favorite toppings—caramelized onions and caramelized mushrooms—into one jammy relish to serve at holiday parties and potlucks. A few other ingredients are added to deepen their flavor, including:
- Yellow onion
- Brown or button mushrooms (known as baby bella mushrooms, too)
- Garlic
- Vermont Creamery Cultured Butter
- Fresh thyme leaves
- Marsala wine
- Kosher salt
- Black pepper
- Sourdough or French bread baguette
Butter Brings Out the Caramelized Jam
If there ever was a scent that signaled the tastebuds to prepare for deliciousness, it would be sizzling butter melting into onions and mushrooms. Butter is what coaxes onions to become tawny gold and feeds thirsty mushrooms the butterfat they need to soften and absorb flavor.
Vermont Creamery Cultured Butter is a butter of a higher standard. What sets their cultured butter apart from the rest is the process. Making cultured butter is like making fine wine, where fermenting the cream as you would grapes for wine—slowly—produces the best flavors. The longer the culture, the better, where after fermenting, wonderful notes of buttermilk and hazelnuts develop in the butter cream for a creamy, delicious flavor.
I used Vermont Creamery Cultured Butter unsalted in this recipe, but their Cultured Butter with Sea Salt works wonders too.
How to Caramelize Onions and Mushrooms
I’ve shared all the ins and outs of how to make caramelized onions here. Adding mushrooms to the mix follows the same cooking methods, but here we’re doing so in a shorter time.
Slice the onions in half from stem to stem, then slice into half moons. Trim the mushrooms and chop or slice, but keep them on the thicker side of things since they’ll cook down considerably and you’ll want them to still be represented.
I use a non-stick cast iron skillet (this one is my favorite) or a heavy bottom skillet so the vegetables deepen and darken and create little bits of frond (the buttery sugar remnants) on the bottom of the pan that are periodically stirred into the whole goodness.
Once the mushrooms and onions are nearly done, I add Marsala wine for a touch of sweetness and to pull up those bits of caramelized goodness from the pan. You shouldn’t worry about these tasting boozy since the alcohol content evaporates as the mixture cooks.
However, if you prefer, alternatives to Marsala wine include Madeira, brandy, or white wine if you prefer. Or, if you don’t want to use alcohol, chicken stock or grape juice will add the needed moisture too.
The final touch is fresh thyme. It compliments and deepens the savory umami of the mushroom/onion/Marsala connection.
How to Toast Crostini
Most grocery stores sell small slices of baguettes and French loaves in their bakery departments, bagged and ready to go. This is a handy time-saving step, but I prefer to slice my own.
To toast, place the cuts of bread on a baking sheet and bake at 400°F for 5 min. I like to lightly brush my crostini with melted butter to seal the savory deal.
Assemble the Crostini
Once toasted and while still warm, spoon the caramelized onion and mushroom mixture directly on the crostini. I like to put two spoonfuls on each toast, then go back and add more once another so I know I have enough.
Sprinkle the toasts with more fresh thyme.
And if you’re feeling like piling on a little creamy goodness, a sprinkling of super creamy goat cheese or a dollop of crème fraîche adds a tang you’ll be happy to savor.
How to Make Crostini Ahead of Time
This is the perfect appetizer to prep ahead of the party and then warm up right before serving.
- Cook the caramelized onion and mushroom mixture then refrigerate for up to 3-4 days ahead of time.
- Slice the baguette one day ahead of serving. Store in an air-tight container or bag, then butter and toast before serving.
- These crostini taste just as good warm or at room temperature. So feel free to set out for nibbles all party long.
More Easy Appetizer Ideas
- Easy Shrimp Cocktail
- Slow Cooker Little Smokies
- French Onion Dip
- The Best Clam Dip Recipe EVER
- Sausage Stuffed Mushrooms With Hollandaise
- Light and Easy Cheese Ball Recipe
- Slow Cooker Sriracha Meatballs
- 15 Holiday Appetizers with 5 Ingredients or Less
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Caramelized Onion and Mushroom Crostini
Ingredients
- 5 tablespoons Vermont Creamery Cultured Butter Unsalted , divided
- 1 medium yellow onion , thinly sliced
- 8 ounces shiitake mushrooms cleaned, stemmed and sliced
- 8 ounces brown mushrooms cleaned, stemmed and sliced
- 1 clove garlic , pressed or minced
- ¼ cup Marsala wine
- 2 teaspoons fresh thyme leaves
- ½ teaspoon kosher salt
- ¼ teaspoon freshly ground black pepper
- 1 loaf baguette
Instructions
- Melt 4 tablespoons of the butter in a large skillet over medium heat. Add the sliced onion and mushrooms and cook for 5 minutes, stirring occasionally, until the onions and mushrooms become tender and golden. Off heat, add the Marsala wine to the onions with the thyme leaves, place back on the burner and reduce the heat to a simmer. Cook for 5-7 minutes or until jammy.
- While the onion mixture cooks, place the sliced bread on a foil or parchment lined baking sheet. Melt the remaining 1 tablespoon of butter in a small saucepan over medium heat or in the microwave on high for 30 seconds. Brush the bread slices with melted butter and toast in the oven until lightly browned.
- To assemble, mound each of the toasts with equal amounts of the caramelized onion mushroom mixture. Sprinkle with more thyme leaves and serve.
Notes
Nutrition
More Crostini Appetizer Recipes
- Melted Mozzarella Caprese Crostini Toasts
- Fig Spread and Cheese Crostini Appetizer
- Pesto and Garlic Shrimp Bruschetta Recipe
- Nectarine and Wisconsin Burrata Bite Crostini
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This post is sponsored by Vermont Creamery. As always, thank you for reading and supporting companies I partner with, which allows me to create more unique content and recipes for you. There may be affiliate links in this post of which I receive a small commission. All opinions are always my own.
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Not Vegan. It contains Butter
Hi Mary! You are correct, this is not a vegan recipe. We have not advertised it as such.
I didn’t have Marsala so I substituted port. Otherwise I followed this recipe. Very yummy! I have leftovers and plan to add as a topping to grilled burgers.
We’re glad you enjoyed this, Jan, and adding the leftovers to grilled burgers is a brilliant idea–yum!!
carmelizing onions takes a long time. hold off on adding mushrooms until onions are tender as mushrooms are very quick to soften.
Was looking for a mushroom, shallot/onion combo that would work well for topping slices of pan-fried polenta, and this recipe was perfect. So good! Will also try it on crostini as well.
Sounds great Palyn!
How do onions carmelize in that short amount of time. I want to make this for a big family gathering, so want to do it correctly.
Hi Judy! We’ve found that they get nice and jammy in that amount of time, but if you find yours aren’t, you can definitely cook them longer to your liking. Let us know what you think – we hope you enjoy!
This looks amazing! I make a bacon jam that would work here as well, but I’ve got to try the mushrooms!
Oh bacon jam would be delish Mike. Happy to hear you’ll give this one a try.
Hello Heidi, this recipe looks amazing, I want to make it for tomorrow, my question is where and when do I add the garlic glove? I had read many times the instructions and the whole explanation above and I can’t find it.. thank you in advance!!
The taste is good…not as great as I hoped for. I could not access the reply to Mariana who asked when to add the garlic, and you have not adjusted the recipe, so I added when I put in the Marsala.
I also don’t know how you managed to fill 16 baguette slices.
Heidi – this sounds really tasty! Two questions for you: first, how thickly do you slice the baguette? I made a crab crostini for a party last week, and that recipe said to make 1/2 inch slices. With fresh baguette, that’s a bit of a challenge, so I figure it’s better to use a day old baguette. What do you do? Second, if you’re making this ahead, do you toast the baguette slices and store them, or toast just before you want to serve the crostini? Thanks!
Thanks El! I cut the baguette in slices about 3/8″ width slices. I used fresh baguette, but since they’re toasted you could use day old if you find that easier to slice. You can for sure slice before hand, then toast them right before serving so they’re still warm and delicious. Enjoy!
Thanks Heidi. I’ll let you know how it works out!!