Boil the pasta for 2 minutes less than stated on the pack, then drain. Meanwhile, melt 2 tbsp butter in a saucepan.
Cook butter until bubbling, then whisk in 3 tbsp plain flour. Cook for 1 minute, then gradually whisk in the milk until you have a lump-free sauce. Stir in Dijon mustard, nutmeg, salt and pepper, then simmer for 5 minutes, whisking all the time until thickened.
Take off the heat, then stir in 250g grated Cheddar, reserve 50g for topping later.
Stir the pasta into the cheesy sauce. Tip half the pasta mixture into a large ovenproof dish, then crumble a layer of Feta across the top. Tip the remaining pasta mixture on top.
Scatter over the remaining 50g grated Cheddar, then bake for 20 minutes until crisp and golden.