Patty Melts

Arrange onion slices on a plate and drizzle vinegar over top. Divide beef into 4 equal portions, shaping each into a 1/2-inch thick patty. Season patties with salt and pepper.

Heat a large grill pan over medium heat. Coat the pan with cooking spray and add onion. Cover and cook for 6 minutes, flipping halfway. Remove from pan, cover, and keep warm.

Heat the grill pan over medium-high heat. Coat with more cooking spray and add patties. Cook until no longer pink in the centers, about 3 minutes per side. An instant-read thermometer inserted into the center should read at least 160 degrees F (71 degrees C). Remove patties to a plate, drain and discard any grease, and wipe the pan clean with a paper towel.

Spread Dijon-mayonnaise blend evenly over all bread slices. Layer 4 of the slices with 2 tablespoons Jarlsberg cheese, 1 beef patty, 2 onion slices, and 2 tablespoons Jarlsberg. Place remaining bread slices mustard-side down on top of the sandwiches.

Heat the grill pan over medium heat and coat with cooking spray. Add sandwiches to the pan and place a cast iron or other heavy skillet on top, pressing gently to flatten sandwiches. Cook until bread is toasted, about 3 minutes per side, leaving the skillet on top as they cook.