Cook the spaghetti in boiling salted water until al dente, drain and set aside approx. 100 ml of the cooking water. Return the spaghetti to the pan, cover and set aside.
Peel and slice the sausage. Slice the onion.
Heat the oil in a frying pan. Sauté the onion and sausage for approx. 1 min. Pour in the cream and reserved cooking water, simmer for approx. 3 mins.
Roughly chop the rocket, add to the sauce along with the cheese, season. Mix the sauce with the spaghetti.