1Melt butter in 7-inch skillet over medium heat. Cook onions, brown sugar and vinegar in butter 20 to 25 minutes, stirring frequently, until onions are golden brown.
2Set oven control to broil. Place baguette slices on ungreased cookie sheet. Broil with tops 4 to 6 inches from heat 1 to 2 minutes or until lightly toasted.
3Spoon about 1 teaspoon caramelized onions evenly onto each baguette slice. Sprinkle with 1/2 teaspoon of the cheese. Broil about 1 minute or until cheese is melted.