These delicious french onion crostinis are warm morsels of caramelized onion and cheesy goodness. Perfectly crispy crostinis topped with caramelized onion, mozzarella, and gruyere and baked to make the perfect appetizer!
I love making these french onion crostinis any time of year, but it's especially a hit in fall and winter when nights are cold and everyone wants something warm to munch on!
This was inspired by my Acorn Squash Crostinis on this site, another delicious fall/winter appetizer.
Ingredients
- Sweet onions
- Baguette
- Mozzarella cheese
- Gruyere cheese
- Olive oil
- Butter
- Dried thyme
- Salt
- Optional for garnish: fresh thyme
See recipe card for quantities.
Instructions
- Step 1: Add olive oil and butter to a heavy bottom hand on medium heat. Add onions and thyme. Cook for about 5 minutes, until onions soften, then add salt. Reduce to medium-low, stirring occasionally, until onions are caramelized (about 50-60 minutes). Remove from heat.
- Step 2: Preheat oven to 425F. Slice baguette diagonally into about ½" thick pieces. Brush both sides with olive oil and sprinkle with salt. Place on a parchment paper lined baking sheet. Bake until crisp and golden, about 5-8 minutes.
- Step 3: Top crostini with caramelized onion.
- Step 4: Next, top with mozzarella and gruyere mixture.
Broil until cheese is completely melted/slightly browned (about 3-5 minutes). Remove from heat, garnish with fresh thyme, and serve.
Hint: Watch the french onion crostinis carefully when you broil them, they can burn quickly!
Substitutions
- Vegan - to make this vegan french onion crostinis, swap out the butter for equal parts olive oil and sub your favorite vegan melting cheese for the mozzarella and gruyere.
- Gluten free - make this recipe with a gluten-free baguette for a completely gluten-free recipe
Storage
You can make the caramelized onions up to a week ahead of time in an airtight container in the refrigerator. I recommend making the crostini and assembling day of.
Top Tip
Making your own crostini really brings this recipe to a different level. I find store-bought crostini are too hard/don't have that softer center that fresh made crostini do.
Related
Looking for other recipes like this? Try these:
French Onion Crostinis
Ingredients
For caramelized onions
- 3 sweet onions halved, thinly sliced
- 2 teaspoon butter
- 2 teaspoon olive oil
- 1 teaspoon dried thyme
- pinch of salt
For crostinis
- 1 baguette
- 2-3 tablespoon olive oil
- pinch of salt
For topping
- ⅔ cup gruyere cheese shredded
- ⅔ cup mozzarella cheese shredded
- optional for garnish: fresh thyme sprigs
Instructions
For caramelized onions
- Heat olive oil and butter in a large skillet on medium heat.
- Add sliced onions and dried thyme and cook, stirring frequently, until softened (about 5 min). Then, add pinch of salt.
- Reduce heat to medium-low and stir every few minutes. It takes about 50-60 minutes for the onions to completely caramelize.
For crostinis
- Preheat oven to 425F.
- Slice baguette diagonally into about ½" thick pieces.
- Brush each side with olive oil and sprinkle with salt.
- Bake until crispy and golden, about 5-8 minutes.
Assembly
- Top crostini with heaping spoonful of caramelized onion.
- Evenly distribute mozzarella and gruyere on top of the crostinis.
- Broil for 3-5 minutes, until cheese is melted and slightly browned.
- Remove from heat and garnish each crostini with sprig of fresh thyme.
Jimmy McFarkle Jr says
Who knew Crostinis could be this good?! Seriously I usually wouldn’t eat fancy food but this is incredible. A work of art. 9000 stars+
Susie McFarkle says
Oh Jimmy now you know where I get all my good recipes from! This ginger lady knows her way around the kitchen.
Don’t tell Margaret about this website, that b couldn’t believe how good my appetizers have been since I found this website. She’s had to sit there quietly and eat while everyone compliments my food! That’s what she gets after years of comments. Payback is sweeter than a Christmas ham!
Kenzie says
Thanks so much for trying them!