Make Onion Sauce - Mix flour into the cooked onions. Cook, stirring, for 1 minute. Add the tomato sauce, water, corn syrup, vinegar, bay leaves, mustard powder, cayenne, and allspice. Stir well to blend and season with salt and pepper. Reduce heat to low, cover, and cook very slowly for 45 minutes. Stir from time-to-time and add a little water if it has become too thick.