Melt butter in pan. Add flour and mix together to form a paste gradually. Add milk till sauce is at the consistency you like, then add cheese keeping some back for topping. Add mustard, salt and pepper; stir.
Drain pasta and add to sauce; stir well. Put in ovenproof dish, sprinkle remaining cheese on top.
Bake for 45 minutes in oven at 180 C / Gas 4.
You can also top with tomato and bacon along with cheese.