Sourdough French Onion Grilled Cheese (Printable Version)

A rich sandwich with caramelized onions, Gruyère, and crisp sourdough bread.

# Needed Ingredients:

→ 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

# 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, approximately 20-25 minutes. Stir in thyme and deglaze with white wine or sherry, scraping up browned bits from the skillet bottom. Cook until liquid evaporates completely and set aside.
02 - Spread Dijon mustard on one side of each bread slice. Top two slices with half the Gruyère and Swiss cheeses, then distribute caramelized onions evenly. 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, approximately 3-4 minutes per side, pressing gently with a spatula to facilitate cheese melting. Remove from heat, rest 1 minute, then slice and serve.

# Expert Advice:

01 -
  • French onion soup flavors in sandwich form — all the richness, none of the spoon required.
  • Deeply caramelized onions add a sweet, jammy complexity that elevates every bite.
  • Two melty cheeses — Gruyère and Swiss — create an irresistibly gooey, nutty filling.
  • Crispy sourdough crust provides tang and texture that perfectly balances the rich filling.
  • Vegetarian-friendly and ready in just 50 minutes from start to finish.
  • Endlessly customizable — swap cheeses, add garlic powder, or pair with tomato soup for a full meal.
02 -
  • Don't rush the onions — true caramelization takes 20–25 minutes over medium heat. Rushing leads to browned but not sweet onions.
  • Grate your own cheese for the best melt. Pre-shredded cheese often contains anti-caking agents that prevent smooth melting.
  • Use medium-low heat when grilling so the bread crisps evenly and the cheese has time to fully melt before the crust burns.
  • Press gently with a spatula while grilling to help seal the sandwich and encourage the cheese to melt into the onion layer.
  • Let the sandwich rest for 1 minute before cutting — this helps the cheese set slightly so it doesn't all spill out when sliced.
  • Deglaze the onion pan with white wine or sherry for a deeper, more complex flavor inspired by classic French onion soup.
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