Preheat the oven to 220C/200C Fan/ Gas Mark7.
Cook the macaroni in water, when it just under done take it off the heat and drain the water.
While the macaroni is cooking make the cheese sauce by melting butter into a sauce pan over low heat. When the butter is foaming add flour and stir till its mixed in.
Slowly add milk stirring constantly with a whisk over low heat. Make sure the milk is fully incorporated before adding any more milk.
Season with salt.=, pepper and mustard. Then add half the cheese, stir to combine and take off heat.
Tip pasta into the sauce and stir till the pasta is fully covered by the sauce. Then pour macaroni cheese into shallow ovenproof dish and sprinkle remaining cheese over the top.
Bake in oven for 10 minutes. The top should be brown and crispy.