Melt the butter in a large pot over medium-low heat. Add the sliced onions and cook slowly for 25 to 30 minutes until deeply caramelized and sweet.

Pour in the beef broth and bring to a simmer.

Add the tortellini and cook for 5 minutes.

Ladle the soup into oven-safe bowls, top with Gruyere cheese, and broil for 2 minutes until the cheese is bubbly and golden.