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.
While bread toasts, coat a large heavy saucepan with cooking spray; add butter, and place over high heat until butter melts.
Add onion, and cook, stirring constantly, 3 minutes or until onion is tender.
Add brown sugar, and cook 5 more minutes or until onion is tender and browned, stirring often.
Spoon onion mixture evenly onto bread slices; top with cheese.
Broil 5½ inches from heat 2 minutes.
Serve immediately.
