Sourdough French Onion Grilled Cheese (Printable version)

A savory sandwich with caramelized onions, melty Gruyère, and crispy sourdough bread.

# 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

→ Sandwich Assembly

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

# Directions:

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. Transfer to a bowl 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 across the cheese layer. Sprinkle with black pepper and top with remaining cheese. Place remaining bread slices on top with mustard side facing inward.
03 - Spread softened butter on the outer surface 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.

# Expert Advice:

01 -
  • The caramelized onions do most of the heavy lifting, giving you that deep, complex flavor you'd normally simmer for hours in broth.
  • Sourdough's tang plays perfectly against the sweetness of the onions and the richness of melted Gruyère—it's like they were meant for each other.
  • This is one of those sandwiches that feels fancy enough to serve guests but easy enough to make on a random Thursday night.
02 -
  • Don't rush the onions—I learned this the hard way by trying to speed things up with higher heat, which just burnt them on the outside while leaving them raw inside.
  • Medium-low heat on the grill is non-negotiable, because medium-high will char your bread before the cheese even thinks about melting.
  • Grating your own cheese instead of using pre-shredded makes a noticeable difference in how smoothly everything melts together.
03 -
  • Save a spoonful of the onion cooking liquid before it evaporates completely and stir it into soft butter before spreading it on the bread—this adds another layer of flavor that people will taste but won't be able to identify.
  • If your bread is very thick, slice it horizontally into thinner pieces so the whole thing stays manageable and the filling stays in proportion to the bread.
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