Bring a large pot of water to a boil and add a big pinch of salt. Cook pasta according to package instructions, reserving ½ cup of the starchy pasta cooking liquid to use later for the sauce.

While pasta is cooking, heat a large, deep skillet over medium-high heat and spray with cooking spray. Add Italian sausage to the pan and begin breaking down using a potato masher or wooden spoon. Cook until sausage is golden brown, stirring frequently, about 7-9 minutes. Once sausage is fully cooked, remove from the pan and set aside.

In the same pan, drizzle in two tablespoons of olive oil and add the onion slices plus a sprinkle of salt and pepper. Reduce heat to medium and cook until onions are golden and caramelized, about 6-8 minutes, stirring frequently.

Once onions are caramelized, return sausage to the pan along with any juices that have formed. Stir in pesto, lemon juice, reserved pasta water, parmesan and chili flakes to create a sauce. Toss in cooked pasta and green peas then stir well. Check for seasoning and adjust to taste before serving. If you have fresh herbs like parsley or basil, sprinkle them in at the end right before serving.