Clean the black cabbage and spring onion and boil them for 15 minutes in boiling salted water. Drain them, being careful not to throw away the cooking water, and blend them together with two tablespoons of oil using an immersion blender. Set aside. Add salt and pepper to taste. Cut the sweet potato into cubes and cook it, together with the pasta, in the same water previously used for the black cabbage and spring onions. Drain the pasta, add the cream of vegetables to it and stir. Finish with flakes of aged strong Provolone Valpadana P.D.O. and serve.