Directions

1. Preheat oven to broiler setting.
2. Melt butter in a saucepan over medium heat. Add onions and cook until softened, 5 to 8 minutes. Stir in the heavy cream and beef broth. Bring to a boil, then reduce heat to low and simmer, loosely covered, for 20 minutes. Season with salt and pepper, to taste.
3. Top each slice of bread with one slice of provolone and one slice of Swiss cheese; place on a cookie sheet. Broil on top oven rack until the cheese browns and the edges of the bread are crispy, 3 to 5 minutes.
4. To serve, ladle soup into 3 bowls and top with one slice of the cheese-topped bread.