1. In a microwave-safe bowl (2 1/2 liter or 2 1/2 quart), combine flour and margarine. Cover with a vented microwave lid and cook on HIGH (1200W) for 40 - 60 seconds.

2. Stir with a wire whisk until smooth. Add 1/2 cup of the milk and stir until mixture becomes creamy. Add the rest of the milk. Cover with a vented microwave lid and cook on HIGH (1200W) for 3 minutes. Add salt and pepper and stir. Cook on HIGH (1200W) for 3 more minutes. Stir until it becomes a creamy white sauce.

3. In a 3-quart casserole dish, add half of the cod, onions, and potatoes. Pour half of the white sauce mixture over the top. Add the rest of the cod, onions and potatoes to the bowl and cover with the remaining white sauce.

4. Cover with a vented microwave lid and cook on HIGH (1200W) for 11 - 12 minutes. When liquid reaches a boil,* reduce power to LOW (30%) and cook for 20 minutes or until the potatoes become soft.

5. Garnish with parsley and serve.

* When using plastic wrap as a cover, poke a hole in the center to release the steam.
* Milk based soup tends to boil over. If it starts to boil over simply remove the cover.

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.