Heat a medium nonstick skillet coated with cooking spray over medium heat. Add onion; cover and cook 5 minutes, stirring occasionally. Add sugar, 1/4 teaspoon salt, and 1/4 teaspoon pepper; cook, uncovered, 5 minutes or until tender, stirring frequently. Remove onion from pan, and keep warm.
Heat pan coated with cooking spray over medium-high heat. Add mushrooms and 1/4 teaspoon salt; saute 5 minutes or until tender.
Combine 1/4 teaspoon salt, 1/4 teaspoon pepper, Worcestershire sauce, and beef. Divide mixture into 4 equal portions, shaping each into a 1/2-inch-thick patty.
Place patties on a grill rack coated with cooking spray; grill 5 minutes on each side or until done. Place bread on grill rack; grill 1 minute on each side or until toasted.
Arrange bread on a baking sheet. Top each bread slice with 1 patty, 1/4 cup onion, 1/4 cup mushrooms, and 2 tablespoons cheese; broil 2 minutes or until cheese melts.
I made this as diet food. Pretty good. I don't think you need to add the sugar or s & p to the onions or the s & p to the mushrooms and both need to cook longer than directed. We grilled the burgers and then did the broiler step and the cheese browned so oven heat would probably work better. I had the rack on the 2nd slot down. I served this with a very spicy slaw - Lexington Style Red Slaw for a low-cal meal.
Delicious! Will definitely make again!! I did a couple things differently I used blue cheese crumbles (reduced fat) instead of Swiss and I prefer no bread so i served bunless with oven roasted potatoes and a spinach salad. Made a quick and super tasty weeknight dinner!