Bring a large saucepan of salted water to the boil. Tip in the pasta and cook for 9–11 minutes until done to your liking, adding the frozen peas for the final minute.
Meanwhile, heat a knob of butter or splash of oil in a frying pan, add the spring onion whites and fry for a minute until just softening. Turn off the heat and set aside until the pasta is cooked.
Drain the pasta and peas, reserving a ladleful of the cooking water, and tip into the frying pan. Add the cream, Parmesan and ham with 3 tablespoons of the pasta water. Stir over a low–medium heat until the cream starts to bubble, then turn the heat down and stir briefly until the sauce is clinging to the pasta. (If you overcook it and it starts looking dry, add a little more of the pasta water.)
Stir in the lemon zest and spring onion greens, then season with salt, pepper and lemon juice to taste. Serve immediately with extra Parmesan.
