How to Make It
Arrange onion slices on a plate. Drizzle vinegar over onion slices. Heat a large grill pan over medium heat. Coat pan with cooking spray. Add onion to pan; cover and cook 3 minutes on each side. Remove from pan; cover and keep warm.
Heat pan over medium-high heat. Coat pan with cooking spray. Divide beef into 4 equal portions, shaping each into a 1/2-inch-thick patty. Sprinkle patties evenly with salt and pepper. Add patties to pan; cook 3 minutes on each side or until done.
Spread about 1 teaspoon mustard blend over 4 bread slices; layer each slice with 2 tablespoons cheese, 1 patty, 2 onion slices, and 2 tablespoons cheese. Spread about 1 teaspoon mustard blend over remaining bread slices; place, mustard side down, on top of sandwiches.
Heat pan over medium heat. Coat pan with cooking spray. Add sandwiches to pan. Place a cast-iron or other heavy skillet on top of sandwiches; press gently to flatten. Cook 3 minutes on each side or until bread is toasted (leave cast-iron skillet on sandwiches while they cook).