Melt butter with oil in heavy large pot over low heat. Add onions, leeks and garlic and cook until onions soften, stirring occasionally, about 15 minutes. Increase heat to medium-low. Add sugar and cook until onions are deep golden brown, stirring frequently, about 30 minutes. Add flour and stir 3 minutes. Add stock, vermouth, hot pepper sauce and pepper. Simmer 40 minutes. Season with salt. (Can be prepared 1 day ahead. Cool; refrigerate. Rewarm before continuing.) Preheat broiler. Divide soup between two 8-oz. soup crocks. Top each with toasted bread and several cheese slices. Broil until cheese melts and top is golden brown. soup 15