How to Make It
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.
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.
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.