Sourdough French Onion Grilled Cheese (Printable Version)

A savory combination of sourdough, caramelized onions, Gruyère, and Swiss melded into a golden grilled delight.

# 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, optional
06 - 2 tablespoons dry white wine or sherry, optional

→ 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

# Step-by-Step Guide:

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.

# Additional Tips::

01 -
  • Those caramelized onions taste like you've been cooking them for hours, but they're done in under 30 minutes once you know the trick.
  • It bridges the gap between a simple grilled cheese and something fancy enough to serve when you want to impress without pretending you slaved away.
  • The sourdough gets impossibly crispy while the cheese inside stays melted and the onions keep their deep, almost wine-like sweetness.
02 -
  • If you try to rush the onion caramelization by turning the heat up, they'll burn on the outside and remain crunchy inside—medium heat and patience create the transformation.
  • Letting the finished sandwich rest for a full minute after cooking lets the cheese set just enough so it doesn't leak out when you bite down, but it's still warm and flowing on the inside.
03 -
  • Slice your onions consistently so they caramelize evenly; a mandoline works beautifully if you have one, but a sharp knife and a few minutes of focus do the job just fine.
  • Toast your bread very lightly before assembling if it's fresh and soft; this prevents sogginess from the onion juices without making it brittle.
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