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" 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.
CALORIES 393 (40% from fat); FAT 17.5 g (sat fat 7.5 g, monofat 6.7 g, polyfat 1.4 g); PROTEIN 31.1 g; CARBOHYDRATE 27.1 g; FIBER 2.5 g; CHOLESTEROL 91 mg; IRON 4.4 mg; SODIUM 786 mg; CALCIUM 204 mg.
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