In a large nonstick skillet, saute onion in 1 tablespoon butter until tender. Remove and keep warm., Shape beef into two oval patties; sprinkle with salt and pepper. In the same skillet, cook patties over medium heat for 3-4 minutes on each side or until a thermometer reads 160° and meat juices run clear; drain. Remove and keep warm., Spread remaining butter over one side of each slice of bread. Place in skillet, buttered side down, and toast until lightly browned. , Combine salad dressing and horseradish. On each of two slices of toast, layer a slice of cheese, a beef patty, and half of the onion and the dressing mixture. Top with remaining toast. ,