Description

There’s something undeniably comforting about a grilled cheese sandwich—the crispy, buttery bread, the gooey, melty cheese. But when you take it up a notch and add caramelized onions inspired by French onion soup? Pure magic.

This French Onion Grilled Cheese Sandwich combines the deep, rich flavors of slowly cooked onions with layers of melted cheese, all tucked between perfectly golden, crispy slices of bread. It’s the kind of sandwich that feels indulgent yet familiar, perfect for cozy afternoons, quick dinners, or a satisfying late-night snack.

If you love the flavors of French onion soup, this recipe brings them together in a handheld form, making it easier and faster to enjoy. Serve it alongside a warm bowl of soup, a crisp green salad, or just enjoy it on its own for a decadent, cheese-filled treat.

Ingredients

Scale

For the Caramelized Onions:

2 large onions, thinly sliced
2 tablespoons butter
1 teaspoon sugar
½ teaspoon salt
½ teaspoon black pepper
½ teaspoon dried thyme
½ cup beef broth

For the Sandwich:

4 slices of sourdough bread (or your favorite bread)
2 tablespoons butter (for spreading)
1 cup shredded Gruyère cheese
1 cup shredded mozzarella cheese
½ teaspoon garlic powder

Instructions

Step 1: Caramelize the Onions

The key to getting deep, rich flavor in this sandwich is perfectly caramelized onions. They take a little time, but the result is worth it!

Heat a large skillet over medium heat and add the butter. Once melted, toss in the sliced onions.
Cook for about 5 minutes, stirring occasionally, until they start to soften.
Sprinkle in the sugar, salt, black pepper, and dried thyme. Stir to coat the onions evenly.
Reduce the heat to low and let the onions cook for 25-30 minutes, stirring occasionally. They should turn a deep golden brown and become jammy.
Pour in the beef broth to deglaze the pan, scraping up any brown bits from the bottom. Let it cook for another 2-3 minutes, then remove from heat.

Step 2: Assemble the Sandwich

Spread a thin layer of butter on one side of each slice of bread. This helps create that crispy, golden crust.
Sprinkle a pinch of garlic powder on the buttered side for extra flavor.
On the unbuttered side of two slices, layer half the shredded Gruyère and mozzarella cheese.
Spoon the caramelized onions evenly over the cheese.
Top with the remaining cheese, then place the second slice of bread on top, buttered side facing out.

Step 3: Grill to Perfection

Heat a clean skillet or griddle over medium-low heat.
Place the sandwiches in the skillet and cook for 3-4 minutes per side, pressing gently with a spatula to ensure even browning.
Once the bread is golden brown and crispy, and the cheese is fully melted, remove from the heat.
Let the sandwiches rest for 1-2 minutes, then slice in half and serve warm.

Notes

Caramelized onions take time – If you’re in a hurry, you can cook them over medium heat for about 15-20 minutes, but for the best flavor, low and slow is the way to go.
Cheese matters – Gruyère is a classic choice because of its rich, nutty flavor and excellent melting quality, but you can experiment with other cheeses like Swiss, provolone, or white cheddar.
Butter alternative – If you want a slightly different flavor, try spreading mayonnaise on the bread instead of butter before grilling. It adds a slight tang and creates an extra crispy crust.
Make it extra indulgent – Add a drizzle of honey or a sprinkle of red pepper flakes for a sweet or spicy twist.
Don’t rush the toasting – Cooking on medium-low heat ensures the bread gets golden and crispy without burning before the cheese melts completely.
Pairing suggestions – This sandwich pairs beautifully with tomato soup, a light green salad, or a warm bowl of French onion soup for a full meal.
Meal prep tip – You can make a big batch of caramelized onions ahead of time and store them in the fridge for up to 4 days or freeze them for up to 3 months to make this sandwich even faster next time.