Preheat broiler. In a large skillet melt butter over medium heat until melted. Add onion; cook for 5 to 7 minutes or until very tender. Stir in French onion soup, the water, and cooked beef. Bring to boiling, stirring occasionally.
Meanwhile, place the bread slices on a baking sheet. Broil about 4 inches from the heat about 1 minute or until toasted on one side. Top the toasted sides of bread slices with shredded cheese. Broil about 1 minute more or until the cheese is melted.
To serve, ladle soup into soup bowls. Top with bread slices, cheese sides up.