Cook the pasta in 4-5 l boiling salted water (approx. 1 teaspoon salt per litre) according to package instructions.
In the meantime, slice the Parmesan cheese into fine shavings. Peel and finely chop the onion. Heat the oil in a pot. Fry the onion until transparent.
Drain the pasta and collect approx. 1⁄8 l pasta water. Put the noodles back into the pot.
Pour cream and pasta water over the onion. Add frozen peas and parsley, bring to the boil and simmer for about 5 minutes. Season with salt and pepper. Mix pea sauce and parmesan well with the pasta.
Season again with salt and pepper.