Cook the pasta according to package instructions in a large pot of salted boiling water until al dente. Reserve 1/2 cup of pasta water, then drain the pasta and set aside.
In a large skillet, heat the olive oil over medium heat. Add the sausage, breaking it apart with a wooden spoon, and cook until browned and cooked through, about 5-7 minutes.
Add the minced garlic and red pepper flakes (if using) to the skillet, cooking for an additional 1-2 minutes until fragrant.
Stir in the frozen peas and cook for another 2-3 minutes until heated through.
Add the cooked pasta to the skillet, along with the reserved pasta water. Toss everything together until well combined. Stir in the grated Parmesan cheese and season with salt and pepper to taste.
Serve immediately, garnished with fresh basil or parsley if desired.