Cook the spaghetti in plenty of boiling salted water.
In the meantime, put the oil in a large pan and fry the onions until translucent, then add the nduja, quartered tomatoes, the chopped garlic and a large handful of basil to the pan. Season with lemon juice, salt and pepper.
When the spaghetti is done, add 2 tablespoons of the cooking water to the sauce, then drain the remaining pasta. Mix the sauce with the spaghetti and add the parmesan on the plate.