Preheat the oven to 400 degrees F.
Line a small baking sheet with parchment paper and spread the pancetta slices on top. Bake until crisp, about 10 minutes.
Divide the ground beef into 6 portions and gently form into patties. Sprinkle the patties generously with salt and pepper.
Oil a grill pan or cast-iron skillet and heat over medium-high heat. Cook the patties 3 to 5 minutes per side for medium doneness. Add the Gruyere slices; cover the patties with a bowl until the cheese is melted. Remove from the grill pan.
To assemble: Place a patty on a bottom bun, top with a piece of crisp pancetta, and finish with the bun top.
Serve immediately, with mustard and cornichons on the side