A French white sauce from which many other sauces descend. Depending upon the food to be sauced, made with part milk, part broth (chicken broth for meat or poultry, fish stock for seafood, vegetable cooking water for eggs, cheese or vegetables).
Melt 3 tablespoons butter in double boiler top over direct moderate heat. Blend in flour, slowly add milk, and heat, stirring, until thickened. Mix in salt and pepper. Set over simmering water. Stir-fry onion in remaining butter for 3-5 minutes over moderate heat until limp; add veal if sauce is for meat or poultry, and stir fry until no longer pink. Add to sauce with remaining ingredients, cover, and cook for 1 hour over simmering water, beating now and then with a whisk and scraping bottom and sides of pan with a rubber spatula. (Note: Be careful when removing lid not to let water condensed there drop into sauce.) Strain sauce, taste for salt and adjust. Serve hot or use as a base for other sauces.