Sourdough French Onion Melt (Printable)

A savory sandwich with caramelized onions, Gruyère, and crisp sourdough for a rich flavor.

# 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 softened unsalted butter
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. 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, approximately 3 to 4 minutes per side, pressing gently with a spatula to facilitate cheese melting. Remove from heat, let rest 1 minute, then slice and serve.

# Expert Tips:

01 -
  • French onion soup flavors in every bite — deeply caramelized onions, melty Gruyère, and a hint of thyme transport you straight to a Parisian bistro.
  • Two cheeses for maximum melt — Gruyère brings nutty richness while Swiss adds creamy, gooey stretch.
  • Crispy sourdough crust provides the perfect tangy, sturdy base that holds up to all the luscious fillings.
  • Vegetarian-friendly and satisfying enough for any main course occasion.
  • Ready in 50 minutes with straightforward steps suitable for a medium-skill home cook.
02 -
  • Low and slow for the onions: Rushing caramelization over high heat will burn rather than sweeten the onions — stick to medium heat and allow the full 20–25 minutes.
  • Grate your own cheese: Pre-shredded cheese contains anti-caking agents that prevent smooth melting. Grating Gruyère and Swiss fresh from the block gives you that perfect gooey pull.
  • Butter the bread, not the pan: Applying softened butter directly to the outside of the bread ensures an even, golden crust without excess grease pooling in the skillet.
  • Press gently: Use a spatula to apply light pressure while grilling — this helps the heat conduct through to the cheese without squishing out the filling.
  • Rest before cutting: Letting the sandwich sit for one minute off heat allows the melted cheese to set slightly so it doesn't all pour out when sliced.
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