Boil 350g of short pasta for 2 minutes less than stated on the pack. Melt 2 tbsp butter in a saucepan.
Add 1 finely chopped garlic clove and 1 tsp english mustard powder, cook for 1 minute, then stir in 3 tbsp plain flour.
Cook for 1 minute, then whisk in 500ml whole milk to make a sauce. Simmer for 5 minutes, whisking until thickened.
Take off the heat, then stir in 250g grated mature cheddar and 25g grated parmesan.