Heat the oil and butter in a soup pot. Add the red onion, yellow onion, shallot and garlic, stir well and cook until the onions are limp and lightly browned. Add the stock and the sherry, bring to a boil, lower to a simmer and cook for about 40 minutes. Drop in the scallions, allow to steep a few more minutes, then ladle the soup into serving bowls. Garnish each bowl with a tablespoon of the parmesan cheese.
Beef broth is a fine substitute for the chicken broth.
For even more flavor, use half chicken broth and half beef broth. All Rights Reserved Reproduction in whole or in part without permission is prohibited.