In A Microwave-Safe Bowl, Heat Butter On High For 45 Seconds Or Until Melted. Stir In Flour Until Smooth. Gradually Stir In Milk. Microwave, Uncovered, On High For 3-4 Minutes Or Until Thickened, Stirring Every Minute. Stir In Beef; Cook On High For 1 Minute Or Until Heated Through. Serve On Toast.