Sauté the onion in the butter until lightly browned. Add the flour and cook over low heat, stirring constantly, until the mixture is light brown in color.
Add the water, vinegar, bay leaf, salt, peppercorns and tomatoes. Simmer for ten minutes. Add the thyme and cayenne and simmer for five minutes longer. Strain. Serve with eggplant dishes, veal dishes, and polenta.