For the soup: In a large pot over medium heat, melt the butter. Add the sliced onions and cook, stirring regularly, until the onions are a rich, caramelized brown. Deglaze the pot with the sherry and cook until the alcohol is cooked off. Add the chicken stock and simmer for 30 to 45 minutes. Season with salt and pepper.
For the plating: Preheat the broiler.
Ladle about 10 to 12 ounces of soup into 6 ovenproof crocks. Top each serving with a crostini followed by 2 slices of Swiss cheese.
Place the crocks on a baking sheet and place under the broiler on the top shelf of the oven. Cook until the cheese is melted and browned.