Cook the pasta in boiling salted water according to the packet instructions.
Meanwhile, heat the oil in a pan and cook the onion for 3-4 minutes until soft.
Add the garlic and cook for 1 minute.
Tip in the frozen peas and 100ml of the pasta cooking water. Cook for 3-4 minutes until the peas are tender.
Drain the pasta, reserving another 100ml of cooking water.
Add the pasta to the pea mixture along with the mint and half the cheese.
Toss everything together, adding splashes of pasta water until you get a light sauce.
Season with salt and pepper and serve topped with the remaining cheese.