Peel onions and slice them into medium sized strings. Set this aside. In a 6 quart or larger stock pot, melt butter over medium low heat and add onions. Using tongs, toss onions in butter to evenly coat. Cook the onions on low heat uncovered for about one hour, stirring occasionally. This slow cooking makes the onions rich, sweet and caramelized. Add salt, pepper, paprika, bay leaves and thyme. Sautee this mixture for 10 minutes over low heat. Pour in beef broth, chicken broth and sherry wine. Increase the heat to medium-high to bring ingredients to a boil. Simmer for another 45 minutes.
Adjust the color of the soup to a rich brown using the kitchen bouquet. Start with one small drop at a time to help obtain the color you desire. While soup simmers, preheat oven to 350 degrees. Once temperature reaches 350 degrees adjust oven temperature to low broil. Slice baguette into ½ inch slices. You will use 2 slices per bowl. Brush the tops with olive if desired. Toast bread for about 3 minutes. Ladle warm soup into oven proof bowl, top soup with two slices of bread and two pieces of cheese per bowl. Heat this under the broiler until the cheese melts and begins to bubble for about 5 minutes-7 minutes. Sprinkle with fresh parsley. Best when served hot out of the oven.