Heat the butter in a large heavy pot over moderate heat and add the onions, separating the rings with a fork. Cover and sauté onions, stirring occasionally, for about 10 minutes. Raise the heat to medium high and cook, stirring often, until the onions are very soft and a deep golden brown, about 20 minutes more. Add beef stock, chicken stock, sherry, brown sugar, pepper, thyme, parsley, paprika and scallions. Simmer for 10 minutes, or longer, if desired. To serve, pour soup into warmed bowls, and top with croutons and Swiss cheese.
To clarify butter: Melt butter in a heavy pan over low heat. Skim off froth and carefully pour clear liquid into another container, being careful to leave the milky residue behind.
Recipe can be halved or doubled.
At the restaurant we served these in onion bowls -- or very large yellow onions with the root cut off and the insides mostly scraped out(this can be used in the soup) with an ice cream scoop. After toping them with croutons and cheese they were placed under the broiler until bubbling.