Meanwhile, heat a large saucepan over medium-high heat, add the pancetta and saute' until crisp and golden brown. Transfer to a paper towel-lined plate to drain. Drain all but 2 tbsp of the grease, add the onions and cook until softened, about 5 minutes. Add the peas and garlic and let cook for another 3 minutes. Add the pasta, pancetta, parmesan cheese and gently toss to combine, adding (a little at a time) some of the reserved pasta water, until pasta is moistened. Serve with extra parmesan cheese.