Begin by boiling a large pan of salted water for your pasta.
In a seperate saucepan heat your butter on a low temperature then gradually add your flour. Stir continuously slowly increasing the heat until you form a paste (roux)
Preheat your oven to 220ºC/425ºF/Gas Mark 7.
Add the macaroni to the boiling water and cook for the time specified on your pasta pack. Once the pasta is fully cooked, drain and add to the saucepan with your sauce in it.
Turn the heat off of the saucepan and stir in the pasta and teaspoon of mustard so that it is fully coated in the sauce. Add the grated cheeses and continue to stir until the cheese begins to melt and become stringy. A wetter consistency is better for your sauce at this stage as it will thicken when baked so a splash of milk if needed can help loosen.
Pour your pasta and sauce into a ceramic dish and bake in the oven for 30 minutes.
When the pasta is almost baked, begin to slice and toast your bread under a grill. If you like you can butter then insides of the toast and once the pasta is done, spoon generous helpings on to your toast, sandwiching two slices together. For an extra cheesey indulgence you can also add some fresh chunks of mozzarella cheese on top for that gooey stringy effect when you slice and seperate the pieces of toast.
For an added touch, why not experiment with other additions in your Mac n Cheese such as bacon or a pinch of chilli powder.