This sandwich gives a nod to the ingredients that compose a French mountain dish called tartiflette. The two stars of the dish are an exquisite cave-aged French mountain cheese called Comté and a rather strong cheese called Reblochon. Don't worry if you don't like strong cheeses, though. Brie or Camembert make fine substitutions. Bacon and onions round out the traditional dish, and usually potatoes do, too. But here, the potatoes are part of the bread.
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Line a plate with paper towels. Cook the bacon in a large nonstck skillet over medium-high heat until crisp. Transfer the bacon to the paper towels to drain. When cool, break each slice in half. Set aside.
Discard all but about 1 tablespoon bacon fat from the skillet. Add the onion and cook over medium heat until soft and beginning to brown around the edges, 8-10 minutes. Salt lightly.
In a small bowl, toss the Comté and cornichons together. Spread butter on one side of each slice of bread. Place 4 slices of bread, buttered side down, on your work surface. Distribute the onion on the bread, followed by the Comté-cornichon mixture and the Reblochon. Place the bacon slices on the cheese and top with the remaining bread slices, buttered side up.
Heat a large nonstick skillet over medium heat for 2 minutes. Put the sandwiches into the pan, cover, and cook for 3-4 minutes, until the undersides are golden brown. Turn the sandwiches, pressing each one firmly with a spatula to compress the filling.
Cover and cook for 3-4 minutes, until the undersides are well browned. Turn the sandwiches once more, press firmly with the spatula again, cook for 1 minute, and remove from the pan. Let cool for 5 minutes. Cut in half and serve.