French Onion Grilled Cheese
Recipe by Grilly CheesePublished 2026-09-12Updated 2026-09-12
Make French onion grilled cheese with slowly caramelized onions, Gruyère, and mozzarella. Includes realistic onion cooking time and sandwich-building tips.
This French onion grilled cheese borrows the sweet onion and nutty cheese combination from French onion soup. The onions take most of the time: let them soften, shrink, and turn amber before building the sandwich. A little water loosens the browned bits without adding a soupy filling.
French Onion Grilled Cheese
Makes 2 sandwiches. Listed times are estimates; visual and temperature cues control doneness.
10 minCook
55 minRest
1 minTotal
66 minYield
2 sandwiches
Equipment
- 12-inch heavy skillet with lid
- Wide spatula
- Wooden spoon
Ingredients
- •4 slices sourdough bread, about 1/2 inch thick
- •3 tablespoons (42 g) unsalted butter, divided: 1 tablespoon for onions and 2 tablespoons softened for bread
- •1 teaspoon olive oil
- •1 large yellow onion (about 300 g), thinly sliced
- •1/8 teaspoon fine salt
- •1/4 teaspoon dried thyme
- •1/8 teaspoon black pepper
- •2 tablespoons water, plus up to 2 tablespoons more as needed to loosen browned bits
- •3 ounces (85 g) Gruyère, freshly grated
- •1 ounce (28 g) low-moisture mozzarella, freshly grated
Instructions
- 1
Start the onions
Melt 1 tablespoon butter with the olive oil over medium-low heat. Add the sliced onion and salt and stir to coat.
- 2
Caramelize patiently
Cook for 35–45 minutes, stirring every few minutes, until the onion is very soft and deep golden. Lower the heat if edges blacken. Add a teaspoon of the extra water at a time when needed to loosen browned bits; let the water evaporate. Timing depends on the pan, burner, and onion moisture.
- 3
Finish the onion filling
Add the thyme, pepper, and the measured 2 tablespoons water. Scrape up the browned bits and cook for about 1 minute, until no free liquid remains. Spread the onions on a plate briefly to release steam.
- 4
Assemble the sandwiches
Spread the remaining 2 tablespoons butter over one side of each bread slice. Mix the cheeses. Place two slices butter-side down, add half the cheese, divide the onions between them, and top with the remaining cheese. Close with the other bread, butter-side out.
- 5
Grill and rest
Wipe out loose onion pieces. Cook the sandwiches over medium-low heat for 4–5 minutes per side, until golden and melted. Rest for 1 minute before slicing.
Tips
- →Browning a few onion edges quickly is not the same as caramelizing the whole onion. Use softness, color, and sweetness as your cues.
- →Do not transfer liquid from the onion pan into the sandwich. Reduce it first.
Variation
Replace Gruyère with Swiss cheese in the same weight. The base recipe uses water rather than beef broth; choose vegetarian-rennet cheeses if you want a vegetarian version.
Questions and answers
Can I caramelize the onions ahead?
Yes. Refrigerate promptly in a shallow covered container and follow the storage guidance below. Reheat only the amount needed before assembling, and account for that time separately.
Can I make the onions in 10 minutes?
You can soften and brown them in that time, but the flavor and texture will differ. This recipe allows a longer gentle cook for evenly caramelized onions.
Storage and reheating
Grilled cheese is best eaten right away. Refrigerate leftovers promptly in a covered, shallow container at 40°F (4°C) or below, within 2 hours of cooking—or within 1 hour when the temperature is above 90°F (32°C). Use within 3–4 days. Reheat to 165°F (74°C), checking the center with a food thermometer. A skillet over low heat, briefly covered and then uncovered, can warm the center and help restore the crust. If using previously cooked chicken or other leftovers, count storage time from when that ingredient was first cooked. See USDA leftovers guidance and FoodSafety.gov minimum temperatures.
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