This soup is an American take on the classic French Onion soup. With the addition of heavy cream, it has a little more fat but lots more flavor!
Preheat oven to broiler setting.
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.
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.
To serve, ladle soup into 3 bowls and top with one slice of the cheese-topped bread.
Per Serving: 1034 calories; protein 24.7g; carbohydrates 34.9g; fat 90.4g; cholesterol 303.7mg; sodium 1166.5mg.