French Onion Grilled Cheese Sandwiches

Ingredients

• 2 mediumyellow onions, peeled, halved and sliced into 6 mm-thick semicircles
• 45 mlunsalted butter, softened, plus extra for buttering the bread
• 15 mlolive oil
• 1 scant teaspoon (about 5 ml)kosher salt
• 1 heaped teaspoon (about 5 ml)fresh thyme leaves
• ½ teaspoon (about 2.5 ml)freshly cracked black pepper
• 1 pinchgranulated sugar
• 45–60 mlbeef stock, beer or white wine, for deglazing the pan
• About 50 g (½ cup), or more to tastefinely grated Gruyère
• 4 slices, or more depending on how many sandwiches you want to makebread

Instructions

1. 1

   Cook the onions

   Set a medium, heavy-based saucepan over a medium heat. Add the butter and olive oil and stir until the butter has melted, then add all the onions and toss to coat. Leave them undisturbed for about 4 minutes. Add the salt, thyme and pepper and stir, then cover the pan and cook the onions in 4-minute intervals, stirring from time to time. Lower the heat if they brown too quickly. They are ready when browned, very soft and jammy.

2. 2

   Deglaze the pan

   Once the onions are fully browned and soft, add the stock, beer, wine or water. Scrape the bottom of the pan with a wooden spoon as the liquid evaporates, about 30 seconds. Take the pan off the heat and leave the onions to rest while you assemble the sandwiches.

3. 3

   Assemble sandwiches

   Butter one side of each slice of bread. On the unbuttered side of a slice, add a generous scattering of Gruyère, some warm onions, then more cheese. Top with another slice of bread, buttered side out, and repeat to make the other sandwich.

4. 4

   Grill and serve

   Cook the sandwiches in a medium frying pan over a medium heat. Turn them over and cook until golden on the outside and melted inside. Transfer to a chopping board and leave to cool for 2–3 minutes before slicing. Serve warm.

What is a French onion grilled cheese?

It is a grilled cheese sandwich filled with onions cooked until soft and deeply browned, along with melted cheese. The idea borrows the familiar onion-and-Gruyère combination of French onion soup and turns it into a handheld meal.

This version relies on patient cooking to make the onions sweet and jammy, while a splash of liquid lifts the browned bits from the pan. Gruyère adds a savoury, melting layer between the onions and crisp bread.

Nutrition and macros by portion

The nutrition label shows one serving as written: 270 calories, 9g protein, 22g carbs, 17g fat.

The bread supplies most of the carbohydrates, while butter and olive oil contribute much of the fat; Gruyère adds fat and protein. Pair a sandwich portion with salad or vegetables for a more balanced plate.

Getting the onions soft and browned

Even slices: Cut the onions into semicircles of roughly the same thickness so they cook at a similar pace. Stirring occasionally helps them soften without scorching.

Gentle heat: Keep the pan at medium, lowering it if the onions are colouring too fast. Covering the pan between stirs helps them cook down; allow enough time for them to become soft and jammy.

Choosing the deglazing liquid

Stock: Beef stock gives the filling a savoury note that recalls French onion soup. Add it once the onions are soft and browned.

Beer or wine: Either can be used instead of stock for a different flavour. Scrape the base of the pan as the liquid reduces so the browned onion residue joins the filling.

Building a sandwich that holds together

Cheese first: A layer of Gruyère against the bread helps bind the onion filling as it melts. Add more cheese on top of the onions, then close the sandwich.

Butter outside: Butter only the outward-facing sides of the bread for a crisp, golden crust. Keep the onion layer contained so it does not spill out while you turn the sandwich.

Judging the finish

Golden bread: Cook over medium heat and turn the sandwiches once the underside is crisp and browned. If the bread darkens before the cheese melts, reduce the heat.

Melted centre: The sandwiches are ready when the cheese is soft and gooey inside. Rest them for 2–3 minutes before slicing so the filling settles slightly.

What to serve alongside

A crisp green salad or lightly dressed leaves add freshness alongside the rich cheese and onions. For a heartier meal, serve with tomato soup or a bowl of vegetable soup.

Recipe notes & tips

• Slice the onions evenly so they soften at a similar rate.
• Lower the heat if the onions begin to brown before they are soft.
• Use a wooden spoon to scrape up the browned bits when adding the deglazing liquid.
• Keep the cheese and onion filling away from the edges to make turning easier.
• Let the sandwiches rest for 2–3 minutes before slicing.

Variations

• Use white wine: Deglaze the onions with white wine for a bright, aromatic note.
• Try beer: Use beer instead of stock for a deeper, malty flavour.
• Change the cheese: Use another good-melting cheese if Gruyère is unavailable.
• Add extra thyme: Increase the thyme slightly for a more pronounced herbal flavour.
• Choose sourdough: Use sourdough bread for a tangy contrast to the sweet onions.

Common mistakes to avoid

• Rushing the onions: They will lack the soft, jammy texture; keep cooking gently until fully tender.
• Letting the onions scorch: Lower the heat when they brown too quickly and stir them occasionally.
• Skipping the scrape: Use the deglazing liquid and a wooden spoon to loosen the browned bits from the pan.
• Overfilling the bread: Too much onion makes the sandwich harder to turn; keep the filling centred.
• Cooking too hot: The bread may brown before the cheese melts; reduce the heat and cook steadily.

How to store

These sandwiches are best eaten warm soon after cooking. If needed, refrigerate leftovers in an airtight container for up to 2 days and reheat in a frying pan over a low-to-medium heat until crisp and warmed through.

Frequently asked questions