• Make-Ahead

Ingredients

1. 1/4 cup extra-virgin olive oil
2. 4 large onions, thinly sliced
3. 1/4 cup fino sherry
4. 1 1/2 teaspoons caraway seeds, lightly toasted
5. Kosher salt and freshly ground pepper
6. 12 slices of rye bread from a square loaf of rye or European pumpernickel bread
7. 1/2 cup whole-grain mustard
8. 10 ounces Gruyère cheese, coarsely shredded

Directions

1. Heat 2 tablespoons of the olive oil in each of 2 large skillets. Add half of the onions to each skillet and cook over moderately low heat for 1 hour, stirring occasionally, or until nicely browned. Scrape all of the onions into 1 skillet. Stir in the sherry and caraway seeds and cook until the sherry is completely absorbed, about 1 minute. Season with salt and pepper. 
2. Preheat the oven to 400°. Lightly spread 1 side of each slice of bread with the mustard and arrange on a large baking sheet. Top with the onions and cheese. Season with pepper. 
3. Bake the open-faced sandwiches for 6 minutes, or until the cheese is melted and the edges of the bread are toasted. Transfer the toasts to a cutting board and cut into triangles.

Make Ahead

The recipe can be prepared through Step 2 up to 4 hours ahead.