1) Heat the oven to 180 degrees. Clean the cauliflower, core it and divide it into florets. Cut the larger florets into slices or in half, to obtain evenly sized pieces. Distribute them in a single layer in a pan lined with baking paper. Season the it takes with a drizzle of oil and a few tufts of rosemary. Add salt and bake for about 40 minutes, stirring a couple of times. When the florets are tender and golden, remove them from the oven.
2) Coarsely chop the hazelnuts, toast them in a non-stick pan without seasoning for a couple of minutes and set aside. Boil the spaghetti in abundant salty water. Meanwhile, remove the gut of the `nduja, chop it up, put it in a large non-stick pan with 4 spoons of oil and heat it over medium heat, mashing with a wooden spoon.
3) Drain the pasta al dente keeping a ladle of its cooking water. Pour it into the pan with the 'nduja adding a few tablespoons of water kept aside to dilute the sauce. Join the cauliflower roasted with its cooking juices and sauté the spaghetti briefly to flavor them. Remove the pan from the heat, divide the pasta into plates and sprinkle with the hazelnuts toasted.