Peel onions and slice 1/8 inch thick, preferably in a food processor. Melt butter in a 6-quart (or larger) stockpot. Add onions; cook, uncovered, over low heat for 1 1/2 hours, stirring occasionally. (The long cooking time makes the onions mellow and sweet.) Stir in pepper, paprika and bay leaf; saute over low heat 10 minutes more, stirring frequently. Pour in 6 cans broth and wine. Increase heat and bring to a boil. Dissolve flour in remaining 1 can broth. Stir into boiling soup. Reduce heat and simmer slowly for 2 hours. Adjust color to a rich brown with caramel coloring, season with salt. Refrigerate overnight. To serve, heat soup in microwave or on stove top. If desired, pour into ovenproof crocks or bowls. Top with a slice of breat and a sprinkling of grated cheese. Heat under the broiler until cheese melts and bubbles, about 5 minutes.