French Onion Grilled Cheese

Featured in: Family Comfort Meals

This sandwich layers deeply caramelized onions with a blend of Gruyère and mozzarella cheeses on buttered sourdough bread. The onions are slowly cooked with butter, sugar, salt, thyme, and balsamic vinegar until golden and flavorful. The sandwich is assembled with mustard and grilled until the bread is crisp and the cheese is melted. Perfectly paired with tomato soup or a fresh salad, this dish offers rich, savory comfort with a French-American twist.

Updated on Fri, 21 Nov 2025 14:05:00 GMT
Golden-brown French Onion Grilled Cheese boasts bubbly cheese and deeply caramelized onions within. Pin this
Golden-brown French Onion Grilled Cheese boasts bubbly cheese and deeply caramelized onions within. | plumoven.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.

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.
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Required Tools

Large skillet, Spatula, Bread knife, Cheese grater

Nutritional Information

Calories 540, Total Fat 32 g, Carbohydrates 44 g, Protein 22 g

Category & Difficulty

Main Dish, Medium difficulty, American French Fusion cuisine, serves 2, Vegetarian

Melted Gruyère and mozzarella oozing from a perfectly toasted French Onion Grilled Cheese sandwich. Pin this
Melted Gruyère and mozzarella oozing from a perfectly toasted French Onion Grilled Cheese sandwich. | plumoven.com

This sandwich brings a comforting and sophisticated flavor to your table perfect for cozy meals.

Recipe FAQs

How do I caramelize onions properly?

Cook thinly sliced onions slowly over medium heat with butter, sugar, salt, and pepper for about 25-30 minutes until golden and deeply caramelized, stirring occasionally to prevent burning.

Can I use other cheeses instead of Gruyère and mozzarella?

Yes, Comté or Swiss cheese can be substituted for Gruyère to maintain a similar creamy melt and flavor profile.

What bread works best for this sandwich?

Sourdough or country-style bread is ideal due to its sturdy texture and ability to crisp up nicely when grilled.

How can I add more depth to the onion flavor?

Add a splash of dry white wine to the onions while cooking for an extra layer of authentic French onion richness.

What are good side pairings for this dish?

Try serving it with tomato soup or a simple green salad for a balanced and satisfying meal.

French Onion Grilled Cheese

A savory combination of caramelized onions and melted Gruyère and mozzarella on toasted sourdough.

Prep time
15 min
Time to cook
40 min
Total Duration
55 min
Created by Plum Oven Sophie Warren


Level of challenge Medium

Cuisine type American-French Fusion

Servings made 2 Portions

Dietary details Meat-Free

Ingredient List

Caramelized Onions

01 2 tablespoons unsalted butter
02 2 large yellow onions, thinly sliced
03 1 teaspoon granulated 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 (approximately 3.5 ounces)
04 1 cup shredded mozzarella cheese (approximately 3.5 ounces)
05 2 teaspoons Dijon mustard

How-To Steps

Step 01

Caramelize the onions: Melt butter in a large skillet over medium heat. Add sliced onions, sugar, salt, and pepper; cook, stirring occasionally, until onions are golden brown and caramelized, about 25 to 30 minutes.

Step 02

Finish onion preparation: Stir in thyme leaves and balsamic vinegar; cook for an additional 2 minutes until aromatic. Remove from heat and set aside.

Step 03

Prepare bread and cheese layers: Butter one side of each bread slice. Spread Dijon mustard evenly on the unbuttered side of two slices.

Step 04

Assemble sandwiches: Layer half the Gruyère and mozzarella over the mustard-spread sides. Add a generous spoonful of caramelized onions atop the cheese, then distribute the remaining cheese. Top with the remaining bread slices, buttered side facing outward.

Step 05

Grill the sandwiches: Heat a clean skillet or griddle over medium-low heat. Cook sandwiches for 3 to 5 minutes per side, pressing gently and flipping once, until the bread is golden brown and the cheese is thoroughly melted.

Step 06

Rest and serve: Allow sandwiches to rest for 1 minute before slicing. Serve warm.

Equipment Needed

  • Large skillet
  • Spatula
  • Bread knife
  • Cheese grater

Allergy Advisories

Review each item for allergens and talk to your healthcare provider if unsure.
  • Contains milk (cheese, butter), wheat (bread), and mustard. Verify store-bought bread for additional allergens.

Nutrition info (per portion)

This nutritional overview is meant for general information. For medical concerns, ask a professional.
  • Caloric Value: 540
  • Fats: 32 g
  • Carbohydrates: 44 g
  • Proteins: 22 g