Here's a roux sauce which you can use for all manner of sauces in general:
Basic recipe:
1oz flour (plain)
1oz marg
1/4pt milk (or more if you want it wetter)
Melt fat in a pan, sprinkle flour over and spread a bit with a fork so it absorbs the fat. Heat gently until the flour begins to look paler and a bit fluffy. Spread out lumps if it's a bit chunky.
Turn the heat right up whilst adding the milk and whizz it all round with a fork so that the flour/fat disperses into the milk. Keep stirring it quickly with the fork while the milk heats. Once it begins to bubble turn the heat right down and allow to simmer for a few moments then it's done.
At this point you can add the cheese top your own tastes, 3oz would be plenty. Or you could add anything else you want in it, or just pour it on to veg and bake.
If you make it thinly with more milk it can be used as a white sauce for lasagne to go between each layer.
Oh and it you master this then you can add 3 egg yolks (off the heat) and then the cheese. Whisk the 3 egg whites until stiff, fold in the yolk mixture and pour into a tall dish and voila, souffle! (Half an hour on GM6, make sure it's low enough in the oven that when it rises it won't stick to the top!)