French Onion Grilled Cheese

A cozy, indulgent sandwich that combines deeply caramelized onions with melted Gruyère (and optional cheese blends), grilled between buttered bread. It’s like French onion soup turned into a gooey, crispy grilled cheese.

Why You’ll Love This Recipe

• Comfort food elevated — sweet, caramelized onions + rich melting cheese

• Layers of flavor and texture in every bite

• Makes good use of slow-cooked onions

• Easy to adapt (bread type, cheese mix, extra ingredients)

• Perfect for when you have a little extra time or want to impress

Ingredients

(Tip: You’ll find the full list of ingredients and measurements in the recipe card below.)

• 4 tablespoons unsalted butter, divided

• 1 pound yellow or Vidalia onions, peeled, halved, and thinly sliced

• Kosher salt and black pepper

• 2 teaspoons sherry, red‑wine or white‑wine vinegar (optional)

• 4 ounces Gruyère cheese, grated

• 4 slices bread (about ½-inch thick)

You may also see variations or optional additions in other versions:

• Dijon or whole-grain mustard

• Thyme or fresh herbs

• Swiss or other melting cheeses

• Bread such as sourdough, rustic country, or whole-grain

Directions

1. In a large skillet over medium-high heat, melt 2 tablespoons butter. Add the onions, season with salt and pepper, and cover. Cook, stirring occasionally, until softened, about 3–5 minutes.

2. Reduce heat to medium-low, uncover, and cook, stirring occasionally, until onions turn deep golden brown (about 20–25 minutes). If they appear dry or start to stick, add a few tablespoons of water, scraping browned bits from the bottom.

3. If using vinegar, pour it in now, loosening stuck bits, and cook until the liquid evaporates, about 1 minute. Transfer onions to a bowl and adjust seasoning.

4. Stir the grated Gruyère into the warm onions.

5. Butter one side of each slice of bread. Place two slices, butter side down, and divide the cheese-onion mixture among them. Top with the remaining bread slices, buttered side up.

6. In the same skillet over medium heat, melt 1 tablespoon butter. Place the sandwiches, cook until the bottoms are golden (3–4 minutes). Add the remaining butter, flip, press gently, and continue until cheese is melted and the second side is golden (another 3–4 minutes).

7. Serve immediately, optionally slicing in half or adding a side (salad, soup, etc.).

Servings and timing

• Serves: 2 sandwiches

• Prep time: ~10 minutes

• Cook time: ~30–35 minutes

• Total time: ~40–45 minutes

Variations

• Use a mix of Gruyère + Swiss or fontina for a milder melt

• Add a spread of mustard or fig jam to cut the richness

• Include fresh herbs (thyme, rosemary) in the onions

• Use sourdough or crusty country bread

• Bake the assembled sandwich briefly in the oven to finish melting

• Add caramelized mushrooms, spinach, or bacon

Storage / Reheating

This sandwich is best eaten fresh. If you have leftovers:

• Wrap tightly and store in the refrigerator for up to 1 day

• Reheat in a 350 °F (175 °C) oven until warmed through — avoid microwaving if possible to preserve crispness

• You may need to press it under foil so the cheese melts again

FAQs

Can I make the caramelized onions ahead of time?

Yes — caramelized onions can be made in advance and stored in an airtight container in the refrigerator for a few days. Rewarm gently before assembling.

What kind of bread works best?

Sturdy bread like sourdough, country-style, or thick-cut artisan loaves are ideal so they hold up to the cold, caramelized onions and melted cheese.

What if my onions burn?

Reduce heat, add a splash of water or wine, and scrape the fond from the pan to prevent burning.

Can I use other cheeses?

Yes — Swiss, fontina, Comté, or a blend with Gruyère work well. Some versions pair Gruyère with Swiss. Gastronotherapy+2Food & Wine+2

Do I have to use vinegar or sherry?

No. The vinegar (or wine) is optional — it deglazes the pan and adds brightness, but the sandwich can still be excellent without it.

How long should onions be cooked?

Typically 20–35 minutes over low to medium heat until deeply golden and soft. Gastronotherapy+3Food & Wine+3Joy the Baker+3

Should I preheat the skillet?

Yes, preheating ensures even browning. Start at moderate heat to prevent burning the bread before the cheese melts.

Can I toast in the oven instead?

Yes — assemble the sandwiches and finish in a 350 °F (175 °C) oven or under the broiler (watch carefully) to melt the cheese.

Is this suitable for vegetarians?

Yes — as long as your butter and cheese are vegetarian-friendly.

Can I add meat (bacon, prosciutto)?

Absolutely! Crisp bacon or prosciutto adds a savory, crunchy element.

Conclusion

French Onion Grilled Cheese is a brilliant twist on two favorites — combining the depth of caramelized onions and the melt of Gruyère into a crispy, savory sandwich. Though it takes a little time, it’s well worth the effort for a lunch or dinner that tastes indulgent yet sophisticated. Let me know if you want me to format it with metric units or tweak any part.

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