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