8 (1/8-inch-thick) slices Vidalia or other sweet onion
1 tablespoon balsamic vinegar
1 pound extralean ground beef
1/4 teaspoon salt
1/4 teaspoon freshly ground black pepper
3 tablespoons creamy mustard blend (such as Dijonnaise)
8 (1-ounce) slices rye bread
1 cup (4 ounces) shredded reduced-fat Jarlsberg cheese
1. 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.
2. 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.
3. 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.
4. 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).