Buttery toasted bread sandwich with melted Gruyère and sharp cheddar, sweet caramelized onions, and tangy Dijon mustard—designed for dunking into tomato soup.
Caramelizing onions takes 15–20 minutes; do not rush. Low heat and patience yield deep sweetness. Slice cheeses thin so they melt evenly and completely before bread over-toasts.
Butter the outside of the bread, not the pan, for even browning and crispy edges. Toast over medium heat; high heat will burn bread before cheese melts.
Ingredients
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Measurement units
- 2 medium yellow onions
- 4 tbsp butter (2 tablespoons plus 2 tablespoons, divided)
- salt
- black pepper
- 4 oz Gruyère cheese
- 4 oz sharp cheddar cheese
- 2 tbsp Dijon mustard
- 4 slices bread (sourdough, brioche, or sturdy white bread)
Ingredients
Scale
Measurement units
- 2 medium yellow onions
- 4 tbsp butter (2 tablespoons plus 2 tablespoons, divided)
- salt
- black pepper
- 4 oz Gruyère cheese
- 4 oz sharp cheddar cheese
- 2 tbsp Dijon mustard
- 4 slices bread (sourdough, brioche, or sturdy white bread)
Instructions
Caramelize the onions
Melt butter (2 tablespoon) in a large skillet over medium-low heat.
Add the sliced onions and a pinch of salt. Stir to coat evenly.
Cook, stirring occasionally, until onions are deep golden brown and very soft, 15–20 minutes. Season with salt and pepper to taste.
Equipment: large skillet or sauté pan, griddle or large skillet
Assemble the sandwiches
Lay out 4 slices of bread on a clean work surface.
Spread Dijon mustard (1 tablespoon) on two slices of bread.
Layer 2 oz Gruyère and 2 oz sharp cheddar on each mustard-spread slice, dividing evenly.
Divide the caramelized onions between the two cheese-topped slices.
Top each with a plain bread slice to form two sandwiches.
Toast the sandwiches
Melt the remaining butter (2 tablespoon). Brush the outside of each sandwich lightly with butter (2 tablespoon) on both sides.
Heat a griddle or large skillet over medium heat until a drop of water sizzles gently.
Place both sandwiches on the griddle and toast until bread is golden brown and crispy, 3–4 minutes.
Flip carefully with a metal spatula and toast the other side until golden and the cheese is fully melted, 3–4 minutes more.
Equipment: griddle or large skillet, metal spatula, large skillet or sauté pan, wooden spoon or silicone spatula
Serve
Transfer sandwiches to a cutting board and cut diagonally in half.
Serve immediately with tomato soup for dunking.
Before you cook
Cook notes
- • Caramelizing onions takes 15–20 minutes; do not rush. Low heat and patience yield deep sweetness.
- • Slice cheeses thin so they melt evenly and completely before bread over-toasts.
- • Butter the outside of the bread, not the pan, for even browning and crispy edges.
- • Toast over medium heat; high heat will burn bread before cheese melts.
- • These sandwiches are designed to be cut in half and dunked into tomato soup.
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