French Onion Grilled Cheese

Featured in: Bite-Size Snacks

This French Onion inspired grilled cheese features deeply caramelized onions paired with melted Gruyère and mozzarella cheeses nestled between buttery sourdough slices. Slow-cooking onions with butter, sugar, and thyme brings out rich sweetness and depth. The sandwich is pan-grilled to golden perfection, making a warm, satisfying dish ideal for lunch or dinner. Optional Parmesan adds extra savoriness while pairing well with tomato soup or crisp greens.

Updated on Fri, 21 Nov 2025 14:27:00 GMT
Golden-brown French Onion Grilled Cheese, oozing with melted Gruyère, and caramelized onions, ready for a delicious bite. Save to Pinterest
Golden-brown French Onion Grilled Cheese, oozing with melted Gruyère, and caramelized onions, ready for a delicious bite. | funcockts.com

A decadent twist on classic grilled cheese, this sandwich combines caramelized onions with gooey melted cheese for a deeply savory, next-level comfort food experience.

I first made this sandwich to elevate a simple grilled cheese and was amazed by how the sweetness of the onions balanced perfectly with the melted cheeses.

Ingredients

  • For the Caramelized Onions: 2 tablespoons unsalted butter 2 large yellow onions, thinly sliced 1 teaspoon sugar 1/2 teaspoon salt 1/4 teaspoon freshly ground black pepper 1 teaspoon fresh thyme leaves (or 1/2 teaspoon dried) 1 tablespoon balsamic vinegar
  • For the Sandwich: 4 slices sourdough or country-style bread 2 tablespoons unsalted butter, softened 1 cup (about 100 g) shredded Gruyère cheese 1 cup (about 100 g) shredded mozzarella cheese 2 teaspoons Dijon mustard

Instructions

Step 1:
Melt 2 tablespoons butter in a large skillet over medium heat Add sliced onions, sugar, salt, and pepper Cook, stirring occasionally, until onions are golden brown and deeply caramelized, about 25 30 minutes
Step 2:
Stir in thyme and balsamic vinegar cook another 2 minutes until fragrant Remove from heat and set aside
Step 3:
Butter one side of each bread slice Spread Dijon mustard on the unbuttered side of two slices
Step 4:
Layer half the Gruyère and mozzarella onto the mustard side Top with a generous spoonful of caramelized onions, then the remaining cheese Place the other bread slices on top, buttered side facing out
Step 5:
Heat a clean skillet or griddle over medium-low heat Cook sandwiches 3 5 minutes per side, pressing gently and flipping once, until bread is golden and cheese is fully melted
Step 6:
Let rest 1 minute before slicing Serve hot
A close-up of a perfectly grilled French Onion Grilled Cheese sandwich, showing the bubbly cheese and toasted bread. Save to Pinterest
A close-up of a perfectly grilled French Onion Grilled Cheese sandwich, showing the bubbly cheese and toasted bread. | funcockts.com

This sandwich always brings my family together for a cozy meal on chilly evenings.

Required Tools

Large skillet Spatula Bread knife Cheese grater

Nutritional Information

Calories 540 Total Fat 32 g Carbohydrates 44 g Protein 22 g per serving

Category and Cuisine

Main Dish Medium Difficulty American-French Fusion Vegetarian Yield 2 servings

Savory and cheesy French Onion Grilled Cheese with caramelized onions; a comforting meal perfect for a cozy dinner. Save to Pinterest
Savory and cheesy French Onion Grilled Cheese with caramelized onions; a comforting meal perfect for a cozy dinner. | funcockts.com

Enjoy this rich and comforting sandwich that’s perfect anytime you crave a gourmet twist on a classic.

French Onion Grilled Cheese

Savory blend of caramelized onions and melted cheese in a warm grilled sandwich.

Prep Duration
15 minutes
Cook Duration
40 minutes
Overall Time
55 minutes
Authored by Funcockts Ethan Miller


Skill Level Medium

Cuisine American-French Fusion

Makes 2 Portions

Diet Facts Meat-Free

What You Need

Caramelized Onions

01 2 tablespoons unsalted butter
02 2 large yellow onions, thinly sliced
03 1 teaspoon sugar
04 1/2 teaspoon salt
05 1/4 teaspoon freshly ground black pepper
06 1 teaspoon fresh thyme leaves or 1/2 teaspoon dried thyme
07 1 tablespoon balsamic vinegar

Sandwich

01 4 slices sourdough or country-style bread
02 2 tablespoons unsalted butter, softened
03 1 cup shredded Gruyère cheese (about 3.5 oz)
04 1 cup shredded mozzarella cheese (about 3.5 oz)
05 2 teaspoons Dijon mustard

Directions

Step 01

Caramelize Onions: Melt 2 tablespoons unsalted butter in a large skillet over medium heat. Add thinly sliced onions, sugar, salt, and pepper. Cook, stirring occasionally, until onions turn golden brown and deeply caramelized, about 25 to 30 minutes.

Step 02

Add Flavor to Onions: Incorporate thyme and balsamic vinegar into the caramelized onions. Continue cooking for 2 more minutes until fragrant, then remove from heat and set aside.

Step 03

Prepare Bread Slices: Butter one side of each bread slice evenly. Spread Dijon mustard on the unbuttered side of two slices.

Step 04

Assemble Sandwiches: Layer half of the shredded Gruyère and mozzarella cheeses on the mustard side of the bread. Top with a generous portion of caramelized onions, then add the remaining cheese. Close sandwiches with the remaining bread slices, buttered sides facing outward.

Step 05

Cook Sandwiches: Heat a clean skillet or griddle over medium-low heat. Cook sandwiches 3 to 5 minutes per side, pressing gently and flipping once, until bread is golden and cheese melts fully.

Step 06

Rest and Serve: Let sandwiches rest for 1 minute before slicing. Serve hot.

Tools Needed

  • Large skillet
  • Spatula
  • Bread knife
  • Cheese grater

Allergy Details

Review all components for allergens and reach out to a health expert if needed.
  • Contains milk (cheese and butter), wheat (bread), and mustard.

Nutrition Details (each serving)

Nutrition details are just for reference and can't replace professional medical counsel.
  • Energy (Calories): 540
  • Fats: 32 g
  • Carbohydrates: 44 g
  • Proteins: 22 g