Sauté onions in butter using 6- to 8-quart uncovered soup pot. Cook over medium heat until onions begin to brown and caramelize. Stir to color evenly and prevent burning. Add brown sugar, decreasing amount if a less sweet soup is desired. Add base if using and mix well. Add strained stock or water, if using base.
Bring to boil, then reduce heat and simmer 25 minutes to develop flavor. Do not overcook onions. Season to taste with salt, if not using beef base, and pepper. Add sherry, if desired.
To make croutons, preheat oven to 350° F.
Cut slices of bread to fit into soup cups. Place on cookie sheet and drizzle with melted butter. Sprinkle with a little garlic powder and generous amount of Parmesan cheese. Bake in preheated oven until cheese is lightly browned. If bread is fresh, keep croutons in a warm place so they will become crisp all the way through.
To serve, spoon soup into individual 8-ounce oven-proof cups. Float crouton on top of each cup of soup. Top each crouton with slice of cheese, making sure slice reaches just over rim of bowl so it will seal down crouton. Top slice of cheese with heaping tablespoon of shredded mixed cheeses.
Set soup cups on tray or cookie sheet and broil in oven 1 to 3 minutes or until cheese melts.
Note: For larger servings, use a larger size bowl and cut both bread and cheese slices to fit. Use only sherry you find acceptable for drinking for this recipe. Do not substitute the cooking variety.Makes 16 side-dish Source: