Sourdough French Onion Sandwich (Printable)

Sourdough and caramelized onions layered with melted Gruyère for a savory, satisfying sandwich.

# What You 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 Make It:

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 to 25 minutes. Stir in thyme and deglaze with white wine or sherry, scraping up any browned bits from the pan bottom. Cook until liquid evaporates completely. Transfer to a plate and set aside.
02 - Spread Dijon mustard on one side of each bread slice. Top two slices with half the Gruyère and Swiss cheeses combined, then evenly distribute the caramelized onions over the cheese. Sprinkle with freshly ground black pepper and top with remaining cheese. Cover each sandwich with a remaining bread slice, mustard side facing inward.
03 - Spread softened butter on the exterior 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 to 4 minutes per side, pressing gently with a spatula to facilitate cheese melting. Remove from heat and let rest for 1 minute before slicing and serving.

# Expert Advice:

01 -
  • Those caramelized onions take time but reward you with deep, sweet complexity that makes every bite feel fancy without fussing.
  • The combination of Gruyère and Swiss cheese creates this creamy, nutty richness that actual soups can't quite match.
  • It comes together in under an hour, so you can serve something that feels restaurant-worthy on a random Tuesday.
02 -
  • The onions truly need those 20-plus minutes; rushing them by cranking up the heat will give you brown onions, not caramelized ones, and there's a real difference in flavor.
  • Don't skip the resting period after grilling—I learned this the hard way when melted cheese immediately poured out onto my plate instead of staying in the sandwich where it belonged.
  • Medium-low heat is your friend when grilling; high heat will char the bread before the inside cheese even softens.
03 -
  • Press gently with your spatula while the sandwich grills—this helps the cheese melt evenly without squishing out the sides.
  • If your onions seem dry during caramelization, add just a tablespoon of water rather than more butter, which keeps them from getting greasy.
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