Sourdough French Onion Grilled Cheese (Print Version)

Savory sourdough sandwich layered with caramelized onions and melted Gruyère cheese, crisped to perfection.

# Ingredient List:

→ 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 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 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 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 aid cheese melting. Remove from heat, let rest 1 minute, then slice and serve.

# Helpful Tips:

01 -
  • It tastes indulgent and restaurant-quality but comes together with everyday ingredients you probably already have.
  • The caramelized onions do most of the heavy lifting, so you get deep, complex flavor without complicated technique.
  • Sourdough's tanginess plays perfectly against the rich cheese and sweet onions, creating a balance that keeps you coming back for more.
02 -
  • Don't skip the caramelization step or try to rush it by turning up the heat—low and slow is what transforms onions from pungent to sweet and complex.
  • The medium-low heat for grilling is crucial because if your skillet is too hot, the bread will char before the cheese melts, so patience here pays dividends.
03 -
  • Use a cast iron skillet or well-seasoned griddle if you have one, as it distributes heat more evenly and creates a more consistent golden crust.
  • Slice your onions by hand rather than using a mandoline—the varied thickness actually helps them caramelize more evenly and develop better texture.
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