Grilled Sourdough Cheddar Melt

from Cooking Light Magazine March 2000

2 tsp butter or stick margarine, divided
cooking spray
2 C coarsely chopped onion
1 lb ground round
1/4 tsp freshly ground black pepper
1/8 tsp salt
4 slices 2% reduced-fat processed sharp cheddar cheese
8 slices sourdough or white bread

Melt 1 teaspoon butter in large nonstick skillet coated with cooking spray over medium-high heat. Add onion; cook for 4 minutes or until golden brown, stirring frequently. Reduce heat, cook 10 minutes or until tender, stirring occasionally. Set aside. Preheat broiler. Combine beef, pepper and salt in medium bowl. Divide beef mixture into 4 equal portions, shaping each into 1/4" thick oval patty. Place patties on a broiler pan coated with cooking spray; broil 4 minutes on each side or until done. Place 1 cheese slice over each of 4 bread slices; top each slice with 1 patty and 3 tablespoons onion mixture. Cover with remaining bread slices. Melt 1/2 teaspoon butter in pan coated with cooking spray over medium heat; add 2 sandwiches to pan. Cook for 4 minutes on each side until browned and cheese melts. Repeat procedure with 1/2 teaspoon butter and remaining sandwiches.