In medium bowl, combine margarine and onions; mix to coat. Sprinkle beef with garlic-pepper blend and salt.
In 3- to 4-quart slow cooker, layer half of the onions, all of the beef and the bell pepper. Top with remaining half of onions. In measuring cup, combine water and bouillon; stir until dissolved. Pour over mixture in slow cooker.
Cover; cook on low setting for 6 to 8 hours.
About 10 minutes before serving, place bun halves, cut side up, on ungreased cookie sheet. With slotted spoon, place beef mixture on bottom halves of buns. Top each with 2 pieces of cheese. Broil 2 to 3 inches from heat until cheese is melted and top halves of buns are toasted. Place top halves over cheese.