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.
This recipe is brought to you by Vermont Creamery
Carmelized onions and mushrooms are my jam. Literally. Jammy. Because 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 add to deepen their flavor, including:
- Yellow onion
- Brown or button mushrooms (known as baby bella mushrooms, too)
- 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 loafs 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 400°F for 5 min. I like to lightly brush my toasts with melted butter to really seal the savory deal.
Assemble the Crostini
Once toasted and while still warm, spoon the caramelized onion and mushroom mixture directly on the toasts. 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 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
If you make this recipe, please let me know! Leave a rating on this recipe below and leave a comment, take a photo and tag me on Instagram with #foodiecrusheats.
Caramelized Onion and Mushroom Crostini
- 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
- 1/4 cup Marsala wine
- 2 teaspoons fresh thyme leaves
- 1/2 teaspoon kosher salt
- 1/4 teaspoon freshly ground black pepper
- 1 loaf baguette
- 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.
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.