Sourdough French Onion Sandwich (Printable)

Sourdough bread layered with caramelized onions, Gruyère, and Swiss cheese for a savory, golden grilled 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:

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 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 caramelized onions. Sprinkle with black pepper and top with remaining cheese. Cover each with 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 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 Hacks:

01 -
  • Those caramelized onions become almost sweet and jammy, nothing like raw onion slices, and they transform a simple grilled cheese into something restaurant-worthy.
  • The combo of Gruyère and Swiss creates this nutty, complex flavor that makes you feel fancy while eating a sandwich at your kitchen counter.
  • Sourdough bread has enough character and structure to hold up to the melted cheese without getting soggy or falling apart.
02 -
  • Don't rush the onion caramelization by turning up the heat, because they'll brown on the outside while staying raw and harsh inside, and you'll end up with bitter bits instead of sweet jammy gold.
  • The mustard isn't optional, even if you think you don't like it, because that sharp tang is what keeps the sandwich from tasting like you're eating melted cheese on bread, turning it into something with actual complexity.
  • If your cheese isn't melting evenly, your heat might be too high or you're not pressing down enough, so lower the temperature slightly and use steady gentle pressure with your spatula.
03 -
  • Make your caramelized onions ahead of time and store them in the fridge, then simply rewarm them before assembling the sandwich, which saves time and lets the flavors deepen.
  • If you add a pinch of garlic powder to the onions during the last minute of cooking, it creates an extra savory dimension without overwhelming the dish.
  • Serve this alongside a simple tomato soup or salad to cut through the richness, or pair it with a crisp Sauvignon Blanc if you're drinking wine.
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