Melt butter in a medium pot over low heat. Add onion and sauté until golden, about 20 minutes. Add water and beef cubes and bring to a boil. Simmer for 20 minutes.

Meanwhile, prepare the croutons. Preheat oven to 450F. Slice bread into cubes. Lay on a lined baking sheet, spray with nonstick cooking spray, and toast for five to seven minutes.

Add croutons over the top of the soup. Layer cheese over the croutons. Simmer for an additional 10 minutes, or until cheese is melted (some of the cheese will melt into the soup and the rest will remain on top). Serve by ladling the soup and some of the crouton/cheese topping into each bowl.