Stilton Burgers

Preheat the grill to medium-high heat.Preheat a large skillet over medium-high heat. Pour the oil into the pan. Add the chopped onion and 1/8 teaspoon salt; cook for 18 minutes or until golden brown, stirring occasionally.Combine 1/8 teaspoon salt, beef, and the following 4 ingredients; mix gently. Divide the mixture into 4 equal portions, forming each into a 1/2-inch thick patty.Place the patties on a grill rack greased with cooking spray; cook for 4 minutes on each side or until desired browning. Sprinkle each patty with 1 tablespoon of cheese for the last 2 minutes of cooking.Place the rolls, cut side down, on the grill rack; cook for 1 minute. Arrange the bottom halves of the rolls on 4 plates; top each serving with 1 lettuce leaf, 1 slice of tomato, 1 patty, 1/4 cup onion mixture, and 1 roll on top.