Bring a large pan of slightly salted water to the boil. Add the pasta and cook for about 10 minutes, following the packet instructions.
Thinly slice the spring onion and garlic.
Heat the oil in a frying pan and sauté the spring onion and garlic for 3 minutes. Add the peas, lemon juice, stir for a few seconds, then add the vegetable stock. Bring to the boil, cover and cook for 3 minutes.
Add the ricotta and stir until it reduces to a thick creamy sauce.
Turn off the heat and stir in the lemon zest and mint leaves.
By now the pasta must be ready. Scoop out some cooking water (about a cupful), then drain the pasta and add it to the ricotta and pea sauce. Combine well, adding some of the cooking water to the sauce if it's too thick.
Serve warm with freshly ground pepper and Parmesan, if liked.