1. Preheat oven to 375°.
2. Arrange bread slices on a baking sheet. Bake at 375° for 8 minutes or until lightly toasted. Remove from oven, and leave bread slices on baking sheet.
3. While bread toasts, coat a large heavy saucepan with cooking spray; add margarine, and place over high heat until margarine melts. Add onion, and cook, stirring constantly, 3 minutes or until onion is tender. Add brown sugar, and cook 5 additional minutes or until onion is tender and browned, stirring often.
4. Preheat broiler.
5. Spoon onion mixture evenly on bread slices; top with cheese. Broil 5 1/2 inches from heat 2 minutes. Serve immediately.