Open-Faced Burgers with Onion-Mushroom Topping

2 teaspoons olive oil
1 medium sweet onion, sliced and separated into rings
2 (8-ounce) packages presliced mushrooms
1/2 teaspoon salt
2 teaspoons balsamic vinegar
1 1/2 tablespoons paprika
1/2 teaspoon salt
1/2 teaspoon dried thyme
1/4 teaspoon ground red pepper
1/4 teaspoon freshly ground black pepper
1 pound ground round
2 English muffins, split and toasted

Preheat grill.
Heat oil in a large nonstick skillet over medium-high heat. Add onion, and cook for 5 minutes or until golden. Add mushrooms and salt; cook 5 minutes, stirring constantly. Add vinegar; remove mixture from pan. Set aside.
Combine the paprika and the next 4 ingredients (paprika through black pepper). Divide the ground round into 4 equal portions, shaping each into a 1/2-inch-thick patty. Coat patties with spice mixture. Grill patties 4 minutes on each side or until done. Place burgers on muffin halves, and top each burger with 1/4 cup onion mixture.
Yield 4 servings (serving size: 1 sandwich)

Nutritional Information
CALORIES 320(30% from fat); FAT 10.5g (sat 3g,mono 4.8g,poly 1.2g); PROTEIN 29.4g; CHOLESTEROL 70mg; CALCIUM 82mg; SODIUM 813mg; FIBER 4.6g; IRON 5.1mg; CARBOHYDRATE 27.3g
Cooking Light, JUNE 2000