In a large kettle or Dutch oven over medium-low heat, melt the butter. Add onions and saute, stirring, until tender. Add the chicken and beef broth, pepper, Gravy Master. Bring to a boil; reduce heat and simmer uncovered for 50 to 60 minutes, stirring occasionally. Add salt, as needed.
Set ovenproof bowls on a baking sheet. Place a thick slice of French bread in each bowl, then ladle soup over the bread. Top with a few thin slices of Gruyere cheese or Swiss.
Place the bowls under the broiler just until cheese is melted and bubbly, about 3 minutes.
Calories per serving, based on about 1/3 cup shredded cheese in each bowl: 297