Rich, savory French Onion Crostini with caramelized onions, melted gruyére, and fresh thyme. An easy to make appetizer recipe that’s perfect for the holidays!

French Onion Soup. It’s one of those things I ALWAYS try when I see it on a menu. I love its rich, deep flavor and warm, comforting feel. And I love how every one tastes just a little bit different!

One of my favorite ways things to do when I’m coming up with new recipes is take flavors I love and use them in new ways. Which is exactly what I did with this French Onion Crostini. It has ALL the flavors of French Onion Soup (including the bread!) but it’s almost like an inside out version.

And all I can say is that if you love French Onion Soup, you’re going to adore this recipe. It’s easy to make, and actually requires very little patience. And the end result is 1000% worth it EVERY TIME.

This crostini is perfect for dinner parties or holiday gatherings and can also double as a light dinner or tasty side dish. The more you make it… the easier it gets! You’re welcome!

what you’ll need for this recipe

Baguette – I used 12 thin slices of French baguette for this recipe. I like using French bread for crostini because it’s narrower and more crusty than Italian bread, which is a bit wider and fluffier. Feel free to use any type of baguette you enjoy. *Note: If you decide on a wider loaf of bread you may need more of the cheese and french onion mixture.

Gruyére cheese – Gruyére cheese is traditionally used in French Onion soup and has a deliciously nutty flavor that pairs well with the caramelized onions. If you can’t find Gruyére, feel free to substitute Emmental, Fontina, Swiss, or Jarlsberg.

Sweet onion – I used one large Vidalia onion, which is a sweeter variety of yellow onion. Regular yellow onions will also work fine for this recipe. See step by step instructions for caramelizing onions below!

Butter – I used 2 Tablespoons of salted butter to cook the onions in. You can also add 1 Tablespoon of olive oil for extra flavor.

Beef broth – Beef broth is the base of French Onion Soup and a key element in creating the delicious French Onion flavor you know and love. Since I don’t add any salt to this recipe, I used regular beef broth. You can feel free to substitute low sodium broth if you prefer.

Cooking sherry – Cooking sherry helps to bring out the beef flavor from the broth, and also adds a touch of sweetness. Definitely DON’T leave the cooking sherry out; it lends a unique flavor to this recipe that can’t be replicated without it!

Sugar – I added 1/2 teaspoon of sugar, which is an optional yet delicious addition to the recipe. This tiny touch of sweetness adds delicious balance to the savory flavors in this crostini.

Flour – I used 1/2 Tablespoon of flour just to thicken the onions slightly. To make this recipe gluten free, omit the flour and use a gluten free baguette.

Worcestershire – 1/2 teaspoon of worcestershire sauce adds another layer of flavor to the onions. A little bit goes a long way, so if you’re tempted to add more, start with the suggested amount and go from there.

Bay leaf & thyme – Finally, I simmered the onions with a bay leaf and used fresh thyme as a garnish. I love using fresh herbs in recipes… they make all the difference!

how to make french onion crostini

Prep the ingredients. Prepare the ingredients by slicing the baguette into 12 thin slices. Cut the onion in half lengthwise (from stem to root) and slice thinly. Cut the cheese into 12 slices.

Caramelize the onions. Melt the butter in a large skillet over medium heat. Add the sliced onions and cook for 5 minutes, stirring constantly, over medium heat. Reduce heat to low, cover, and cook for an additional 10-15 minutes or until golden brown. Stir onions periodically to prevent burning.

Add the remaining ingredients and reduce. To the onions, add the broth, cooking sherry, sugar, flour, worcestershire sauce, and bay leaf. Bring to a boil, then reduce heat to medium (mixture will still be at a low boil). Cook to reduce until most of the liquid has evaporated.

Toast the baguette and cheese. While the onion mixture is reducing, top the baguette slices with cheese and place on a rimmed baking sheet. Bake at 350°F for 6-8 minutes or until cheese is melted and bubbly.

Assemble the crostini. Divide the onion mixture evenly among the crostini; garnish with fresh thyme and fresh cracked pepper.

how to caramelize onions

Melt the butter in a large sauté pan over medium heat.
Add the sliced onions and season with salt & pepper as desired. Cook, stirring constantly, for 5 minutes. The onions should start to turn golden in color.
After 5 minutes, reduce the heat to low and cover the pan. Continue to cook for 10-15 minutes, stirring the onions occasionally to prevent sticking.
Once the onions are deep golden brown in color, remove from heat and use as desired.

Pro Tips

Slice your baguette in advance and store it in an airtight container or ziploc bag.
Slice your onions in advance and store them in an airtight container in the refrigerator.
Be sure to cook your onions low & slow (see “how to caramelize onions” above)
If you want a LOT of onions on your crostini, make double the onions. They cook down quite a bit!
For best flavor, serve immediately after assembling.

Recipe Variations

Use whole wheat, sourdough, or any type of baguette you prefer
Substitute regular yellow onions for sweet onions
Substitute Emmental, Fontina, Swiss, or Jarlsberg cheese for Gruyére.
Add sliced or quartered button mushrooms for a delicious twist
Add chopped bacon for extra flavor
Use any fresh herbs as a garnish
Sprinkle with fresh shaved Parmesan cheese

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