Heat the oven to 200C/180C fan/gas 6.
Boil the pasta in a large saucepan until it is al dente.
Melt butter in a saucepan.
Add the garlic and cook for 1 min.
Stir in the plain flour and cook for a minute more, then gradually whisk in the milk.
Simmer the sauce, continually stirring, until it thickens.
Take the saucepan off the heat and stir in 250g of the cheese.
Stir in the creamy cheese sauce into the pasta
Pour the pasta mix into a large ovenproof or casserole dish
Scatter over the crushed nachos, jalapeƱos and the rest of the grated cheese.
Bake until the cheese has melted.