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.