1. In a deep 1-quart, heat-resistant, non-metallic casserole melt butter in Microwave oven 30 seconds. 2. Blend in flour and salt until smooth. 3. Gradually, stirring constantly, add milk. 4. Heat, uncovered, in Microwave Oven 2 to 3 minutes or until thickened and smooth. Stir frequently. Makes 1 cup Variations. Thin White Sauce: Follow recipe for Medium White Sauce reducing butter and flour to 1 tablespoon each. Thick White Sauce: Follow recipe for Medium White Sauce increasing butter and flour to 3 to 4 tablespoons each depending upon desired thickness.