Make a roux; 1 T of margarine and 1 T flour, cook until sizzling but not burnt.
Gradually add the milk stirring to prevent lumping.
Add the grated cheese and stir until melted.
Layer potatoes and onion, bacon mix in a microwave proof container.
Pour the cheese sauce over the layered potatoes.
Cover and microwave until a skewer slides easily through the layers.
NB: Cooking times will vary depending on your microwave wattage. Mine is an oldy 750 watt. It took 40 minutes to cook (I used very, very large potatoes) I also turn the mixture over and the half way mark to speed the process further (grumbles from the folks). I also used an 2 litre ice cream container which has a no. 5 recycle mark.