French Onion Grilled Cheese Sandwich

This French Onion Grilled Cheese Sandwich with caramelized onions, melty Gruyere and toasty bread is wholesome comfort food! A tasty vegetarian meal, or make it vegan with vegan cheese!

Every once in a while, I get a hankering for a gooey, melty, toasty grilled cheese sandwich. They tick all the boxes for me as far as “comfort food,” and I often add other things besides cheese to jazz them up, like in our Caprese Grilled Cheese with summer tomatoes and arugula pesto or this Grilled Cheese with Cranberry Mustard and leftover Thanksgiving turkey.  

But winter is here, and I crave umami depth, so I caramelized some onions and added them to the mix. Over the top delicious!

Why You’ll Love This

It’s so comforting! And sometimes I just need that. Flavorful, nutty Gruyere cheese is melted over onions that are caramelized long and slow, prepared like traditional French Onion Soup. Vegans can sub a melty vegan cheese!

Layers of delicious flavor! The process of caramelizing the onions brings out their delicious complexity and sweetness. I prefer to use a wholesome, homemade sourdough bread if possible, but in a pinch, store-bought French bread works great too.

Ingredients

• Onion: Use a large red onion, sweet onion, or yellow onion. These are the best for caramelizing!
• Gruyere cheese: We prefer to buy a block of cheese to grate, but you can use sliced gruyere too. To make vegan, sub a meltable vegan cheese.
• Hearty bread: Try our Sourdough Bread, or buy French bread or another rustic loaf.
• Butter and olive oil: Or use just olive oil. We like to use a mixture of unsalted butter and olive oil to bring the bread to golden-brown perfection.
• Sherry cooking wine (optional): Or use port, red-wine, or beef stock. You could also sub a splash of balsamic vinegar. For depth and complexity, infusing the onions with rich flavor.
• Fresh thyme (optional): For an especially woodsy, savory flavor that makes this grilled cheese perfect in winter.
• Whole grain mustard (optional): To give this grilled cheese a bit of sharp, pungent flavor that really elevates.

Buying Gruyère Cheese

Gruyere was always the cheese that was too expensive and luxurious to buy in my younger years. These days, I splurge on occasion, when I know it will make a real difference. You could substitute another cheese, and even make this vegan with all the good melty vegan cheeses available!

But I prefer Gruyere here – it’s earthy, complex and nutty, with the perfect balance of sweetness and saltiness, pairing perfectly with the caramelized onions.

For the full-flavored experience, splurge a little and try Gruyere. It will rock your world.

For this recipe, make sure to buy Gruyere that is young and softer, rather than aged… which will be hard with visible cracks.

How to Make French Onion Grilled Cheese

Step one: Warm olive oil over medium heat in a large skillet or cast iron pan. Add the sliced onion and stir, then reduce heat to medium-low. Sauté 20 minutes, stirring occasionally.

Step two: Once the onions sweat out their moisture and turn golden brown, add a generous pinch of kosher salt and a splash of sherry, port, or stock (optional), letting the liquid cook off—this is the essence of French onion soup! Turn off the heat, then remove the onions from the pan. Wipe with a paper towel to use again. Preheat the oven to 350F.

Step three: Spread the slices of sourdough (or French bread) with whole grain mustard (optional). On two slices, divide and layer the grated Gruyere, then add caramelized onions, sprinkle with fresh thyme leaves, and finally, add the rest of the cheese. Top with the other slice of bread and gently press together.

Step four: Bring the pan back to medium-low heat, then add the butter and olive oil. Place the sandwiches in the skillet and toast. Once the bread is golden brown and the cheese has begun to melt, carefully flip the sandwich to toast the other side.

Step five: If using an ovenproof skillet, simply place in the oven to fully melt the cheese. Otherwise you can transfer the sandwiches to a baking sheet. This prevents the bread from getting too dark.

Cut in half and enjoy immediately!

Chef’s Tips

1. Grate the cheese. Freshly grated cheese is softer and melts more evenly, with a creamier consistency.
2. Use an ovenproof skillet. This way you can simply place the pan straight into the oven to let the cheese melt without letting the bread get too dark.
3. Use a combination of butter and olive oil. Butter adds rich, creamy flavor, while the oil prevents the bread from burning and turns the bread perfectly golden brown.

Storage

French Onion Grilled Cheese is best enjoyed right away, however you can store leftovers in the refrigerator for 2-3 days. Wrap tightly in plastic wrap to store. To reheat, place on a sheet pan in a 400F oven until warm all the way through, about 5-7 minutes.

Serving Suggestions

Serve this with a green salad, dressed with a light Dijon vinaigrette and enjoy the luscious flavors and textures!

Hope you love this delicious recipe as much as we do! Let me know what you think of the French Onion Grilled Cheese Sandwich in the comments!

xoxoxo

Sylvia

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