In a saucepan cook the onion in the butter over moderately low heat, stirring, until it is softened. Stir in the flour and cook the roux, stirring, for 3 minutes. Add the milk in a stream, whisking vigorously until the mixture is thick and smooth, add the salt and the white pepper, and simmer the sauce for 10 to 15 minutes, or until it is thickened to the desired consistency. Strain the sauce through a fine sieve into a bowl and cover the surface with a buttered round of wax paper to prevent a skin from forming. Makes about 2-1/4 cups. sauce 12