Dinner

French Onion Grilled Cheese Recipe

Name: French Onion Grilled Cheese Recipe
Prep time: 10 min
Cook time: 35 min
Total time: 45 min
Cuisine: French
Ingredients: 2 tablespoons unsalted butter; 2 large yellow onions, thinly sliced; 1 teaspoon granulated sugar; 1/2 teaspoon salt; 1/4 teaspoon freshly ground black pepper; 1 teaspoon fresh thyme leaves or 1/2 teaspoon dried thyme; 1 tablespoon balsamic vinegar; 4 slices sourdough or country-style bread; 2 tablespoons unsalted butter, softened; 1 cup shredded Gruyère cheese (approximately 3.5 ounces); 1 cup shredded mozzarella cheese (approximately 3.5 ounces); 2 teaspoons Dijon mustard

ByLuna Ryder January 10, 2026January 10, 2026

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Introduction

French Onion Grilled Cheese takes the classic grilled cheese sandwich to a whole new level with sweet, caramelized onions and a blend of melted cheeses. This rich and savory twist makes for a comforting meal that’s perfect any time of day.

A grilled sandwich cut into two stacked halves sits on a white plate on a white marbled surface. Each sandwich half has two layers of golden-brown toasted bread with a crispy texture, dotted with small herbs. Between the bread layers, there is a thick, gooey layer of melted white cheese that stretches slightly out of the sandwich, and beneath the cheese, there is a layer of soft, cooked, translucent caramelized onions. Tiny green herb leaves are scattered on and around the sandwich. Photo taken with an iphone --ar 4:5 --v 7

Ingredients

2 tablespoons unsalted butter
2 large yellow onions, thinly sliced
1 teaspoon granulated sugar
1/2 teaspoon salt
1/4 teaspoon freshly ground black pepper
1 teaspoon fresh thyme leaves or 1/2 teaspoon dried thyme
1 tablespoon balsamic vinegar
4 slices sourdough or country-style bread
2 tablespoons unsalted butter, softened
1 cup shredded Gruyère cheese (approximately 3.5 ounces)
1 cup shredded mozzarella cheese (approximately 3.5 ounces)
2 teaspoons Dijon mustard

Instructions

Step 1: In a large skillet over medium heat, melt 2 tablespoons unsalted butter. Add the sliced onions, granulated sugar, salt, and black pepper. Cook, stirring occasionally, until the onions are golden brown and deeply caramelized, about 25 to 30 minutes.
Step 2: Stir in the thyme leaves and balsamic vinegar. Cook for an additional 2 minutes until fragrant, then remove the skillet from heat and set aside.
Step 3: Butter one side of each bread slice evenly. Spread Dijon mustard on the unbuttered side of two slices.
Step 4: On the mustard-covered side of the bread, layer half of the Gruyère and mozzarella cheeses. Add a generous spoonful of caramelized onions on top, then cover with the remaining cheese. Place the remaining bread slices on top, buttered side facing outward.
Step 5: Heat a clean skillet or griddle over medium-low heat. Place the sandwiches in the skillet and cook for 3 to 5 minutes per side, pressing gently and flipping once, until the bread is golden brown and the cheese is fully melted.
Step 6: Allow the sandwiches to rest for 1 minute before slicing. Serve warm.

Tips & Variations

For extra flavor, try adding a sprinkle of fresh herbs like chives or parsley when assembling the sandwich.
Use a mix of Gruyère and sharp cheddar for a different cheese profile.
To speed up caramelizing onions, add a splash of water during cooking to prevent burning and help soften.

Storage

Store leftover sandwiches wrapped tightly in foil or an airtight container in the refrigerator for up to 2 days. Reheat gently in a skillet over low heat or in a toaster oven to keep the bread crispy and cheese melty.

How to Serve

A grilled sandwich cut diagonally into two halves stacked on a white plate sits against a white marbled texture background. Each sandwich half shows three clear layers: the top and bottom layers are golden-brown toasted bread with visible texture and specks of herbs, the middle layer is melted white cheese that oozes slightly out of the bread, and between the cheese layers there are caramelized onions with a shiny, soft texture and a light brown color. Small fresh green herb leaves are scattered on the bread and some on the plate for garnish. The photo taken with an iphone --ar 4:5 --v 7

Serve this delicious recipe with your favorite sides.

FAQs

Can I use different types of onions for this recipe?

Yes, yellow onions are preferred for their natural sweetness and texture, but you can also use white or sweet onions. Keep in mind cooking times may vary slightly depending on the onion type.