- Melt the butter in a large, heavy pot over medium-high heat. 
- Add all the onions, salt, and pepper. 
- Cook, stirring often, until the onions become a deep golden brown. This takes about 15 to 18 minutes. 
- Pour in the sherry and allow it to reduce slightly while stirring. 
- This will take around 2 to 3 minutes. 
- Add the chicken and veal stocks. 
- Bring the mixture to a boil, then reduce the heat to low and let it simmer. 
- Allow this to cook for about 1 hour, letting all the flavors develop. 
- Preheat your oven to 300 degrees F. 
- Put the French bread croutons on a baking sheet and toast them in the oven until they are dry. 
- This should take about 45 minutes. 
- Ladle the hot soup into broiler-safe bowls. 
- Top each bowl with toasted croutons and plenty of grated Swiss cheese. 
- Broil the bowls until the cheese is bubbly and golden brown. 
- This takes about 2 to 3 minutes.