Bring a large pan of salted water to the boil and cook pasta until tender with a bite at the centre, about 8 minutes. Meanwhile put a large heatproof bowl over a pan of water and melt butter. Stir in milk. Add flour.
Add 200g of the cheese, mustard and lemon juice and simmer until cheese has melted.
Add cooked pasta into the same bowl and mix together with cheese sauce.
Put into an ovenproof dish and top with the remaining 50g cheese.
Bake in the preheated oven until heated through, about 35 minutes.