I am wondering whether I can microwave Nigella's Mac And Cheese recipe, or whether its best served out of the oven?
Nigella's Macaroni Cheese recipe (from Nigella Express and on the Nigella website) is designed to be fairly quick and is cooked in a very hot oven for only 15 minutes. The cooking time is fairly short as the pasta is already hot and the sauce takes very little time to heat through. It would be possible to heat the dish straight after assembly in a microwave, with an appropriate cover over it, but you will not get the blistered top and slightly crunchy edges that you get from the hot oven, and in our opinion it will not be quite as good.
Leftovers could be reheated in individual portions in the microwave. If you are making the Macaroni Cheese ahead then you may find that the pasta absorbs a fair amount of the liquid that is in the dish. For a good make-ahead mac and cheese recipe we would suggest trying the Deluxe macaroni Cheese recipe from Nigella Christmas or the Mini Macaroni Cheese (which can be made in a larger dish) from Nigellissima. Both of these recipes can be made in advance and heated when needed.