Preheat the oven to 220°C/ 200°C fan/ gas mark 7/ 425°F
Mix the milk and evaporated milk, to a total of 1000ml. Heat in a microwave or saucepan to below boiling point
Grate both cheeses (a coarse grate is okay). Put aside about 50g cheese for the topping
Heat a large pan of water to cook the macaroni
For the Cheese Sauce:
Melt the butter in a large saucepan over a medium heat. Add the flour and cook for 1-2 minutes
Add about a quarter of the milk and stir well as it all starts to bubble. Once you have a solid mixture add the next quarter litre of milk and repeat until all the milk is used up.If it’s lumpy, use the stick blender to smooth it out
Add the mustard and nutmeg. Add 300g grated cheese, stirring, and taste as you add salt
When the cheese has melted and the sauce is smooth, take off the heat. Put a lid on to stop a skin forming
For the Macaroni Cheese:
Cook the macaroni in boiling, salted water for two minutes less than the packet says. Drain
If there is room in your pan mix the macaroni and cheese sauce. If not, tip the macaroni into the gratin dish, pour the sauce on top and mix carefully
Sprinkle over the reserved cheese
Cook for 20-25 minutes or until the top is browning and the sauce is beginning to bubble
Leave to stand for about 20 minutes out of the oven before serving.