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Put the pasta in a ceramic or glass microwavable bowl. Pour over 700ml boiling water and cook for 2 mins on high; stir and repeat twice more (6 mins in total). Drain the pasta and return it to the bowl.
Put the cornflour and 1 tbsp milk into a jug and stir to make a paste. Add the remaining milk and mix well. Pour over the pasta.
Mix in the mustard, along with the grated gruyère and cheddar, and stir well. You can cook the pasta in the same bowl or tip it into a microwavable serving dish.
Cook on high for 5 mins, stirring halfway through. Sprinkle over the parmesan and dot with small pieces of butter. Cook for a further 2 mins, then serve with a good grating of black pepper, if you like.