1) Bring a large pot of salted water to the boil over a medium heat. Add the pasta and cook until al dente.
2) Meanwhile, saute the pancetta in a large saucepan over a medium-high heat until it is golden and crisp, about 6 minutes. Transfer to a paper towel-lined plate.
3) In the same pan, saute the onions until softened, about 5 minutes. Add the peas and garlic and saute for 3 minutes. Stir in the Parmesan and pancetta.
4) Drain the pasta, reserving 250ml of the pasta water. Add the pasta to the other ingredients, moistening with some of the reserved pasta water if necessary.
Toss to mix, season with salt and pepper, if necessary, and serve sprinkled with lemon juice.