Sourdough French Onion Grilled Cheese (Print version)

A savory sourdough sandwich packed with caramelized onions and melted Gruyère cheese.

# What You'll Need:

→ Caramelized Onions

01 - 2 tablespoons unsalted butter
02 - 2 large yellow onions, thinly sliced
03 - 1 teaspoon sugar
04 - 1/2 teaspoon salt
05 - 1 teaspoon fresh thyme leaves
06 - 2 tablespoons dry white wine or sherry

→ Sandwiches

07 - 4 slices sourdough bread
08 - 1 tablespoon Dijon mustard
09 - 1 cup grated Gruyère cheese
10 - 1/2 cup grated Swiss cheese
11 - 2 tablespoons unsalted butter, softened
12 - Freshly ground black pepper to taste

# How-To Steps:

01 - Melt 2 tablespoons butter in a large skillet over medium heat. Add sliced onions, sugar, and salt. Cook, stirring frequently, until onions are golden and deeply caramelized, about 20 to 25 minutes. Stir in thyme and deglaze with white wine or sherry, scraping up any browned bits. Cook until liquid evaporates completely. Set aside.
02 - Spread Dijon mustard on one side of each bread slice. Top two slices with half the Gruyère and Swiss cheeses, then evenly distribute the caramelized onions. Sprinkle with black pepper and top with remaining cheese. Cover each with a remaining bread slice, mustard side facing inward.
03 - Spread softened butter on the outside of each sandwich. Heat a skillet or griddle over medium-low heat. Place sandwiches in the skillet and cook until golden brown and crisp, about 3 to 4 minutes per side, pressing gently with a spatula to help cheese melt. Remove from heat, let rest 1 minute, then slice and serve.

# Tips from the pros:

01 -
  • Those caramelized onions bring a sweet-savory depth that makes every bite feel restaurant-worthy without actually requiring restaurant-level skill.
  • The contrast between the crispy, buttery exterior and the molten cheese interior is genuinely addictive in the most satisfying way.
  • It transforms humble ingredients into something that tastes fancy enough to serve when people are coming over, but it's simple enough to make on a random weeknight.
02 -
  • Don't rush the onions by cranking up the heat—high heat makes them brown too fast on the outside while staying raw inside, and you lose all that deep caramel sweetness.
  • The temperature of your skillet when you add the sandwiches matters enormously; too hot and the bread burns before the cheese melts, too cool and you end up with buttery but not crispy results.
  • Slicing on the diagonal isn't just for show—it actually makes the sandwich easier to hold and somehow makes every bite feel more intentional.
03 -
  • Make your caramelized onions ahead of time and store them in the fridge for up to five days; this way you can have these sandwiches ready in 10 minutes whenever the craving hits.
  • If your skillet tends to have hot spots, rotate your sandwiches halfway through cooking to ensure even browning on both sides.
  • Don't slice into the sandwich immediately after removing it from the heat—that one minute of resting lets the cheese firm up just enough to stay put instead of running all over your plate.
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