Cook the pasta for around 8 minutes or 2-3 minutes less than the packet directs until really al dente. Scoop out a load of the pasta water and then drain the pasta. Cool under the cold tap and then leave to one side in a colander to drain. Drizzle the oil onto the pasta and toss it to stop it sticking.
Meanwhile, preheat the grill to high and melt the butter together with the nduja in a large saucepan over a medium heat. Add the flour and mix until claggy. Whisk in the milk, a little at a time until smooth. Cook out the flour in the sauce for 10 minutes on a super low heat, whisking every couple of minutes so the sauce doesn't go lumpy.
Chuck 100g of each cheese into the sauce and add plenty of salt and pepper. Mix well and cook for a few minutes, stirring occasionally, until the cheese has melted.
Whisk in 250ml of the pasta water so that the sauce gets thinner and glossier. Add the pasta and mix well. You want it to be creamy with plenty of sauce. Tip into a baking dish and scatter over the remaining cheese. Pop under the grill for 6-8 minutes or until the top is golden and bubbling. Serve immediately.
