Cut the spring onions into thin rings and quarter the mushrooms. Pluck the parsley leaves from the stalks and chop finely.
Heat the butter in a pan and fry the mushrooms vigorously for 5 minutes. Add spring onions and fry for 5 minutes. Deglaze with vegetable stock and bring to the boil.
Cut the cheese into small cubes and mix with the creme fraiche. Add the chopped parsley and stir everything into the sauce. Season the sauce with salt and pepper.
Cook the pasta in plenty of salted water until al dente, then drain. Mix with the cheese and mushroom sauce and serve immediately.