1. Place the flour and butter in a microwave-safe bowl (2 1/2 liter or 1 1/2 quart). Cover with a vented microwave lid and cook on HIGH (1200W) for 40 to 60 seconds to melt the margarine.

2. Take out and mix with a wire whisk until it becomes pasty. Add 1/2 cup of milk and stir with a wire whisk until it becomes creamy. Add the remaining milk and mix. Cover and cook on HIGH (1200W) for 2 minutes. Take out, add salt and pepper and cook on HIGH (1200W) for 2 more minutes. To finalize, stir until the sauce becomes creamy and smooth.

* <White Sauce> can be used for chowder, casserole, sauce for fish dish, or chicken dish.

For how long you should cook with your microwave oven, see Cooking Time vs Wattage Conversion.
For more tips of using Microwave Oven, see Sachiko's Guideline to Microwave Cooking.