Bring a large pot of salted water to a boil and cook the rigatoni according to package directions until al dente; drain and set aside.
Meanwhile, heat the olive oil in a large high-sided skillet over medium-high heat.
Add the ground Italian sausage and diced onion; cook 5–6 minutes, breaking up the sausage and stirring frequently, until the sausage is browned and onions are softened.
Add the minced garlic and cook 1 minute, stirring constantly; if desired, spoon off excess fat but some will add flavor.
Stir in the grape tomatoes and chicken broth and sauté about 4 minutes, until the tomatoes soften and begin to burst.
Add the zucchini, crushed tomatoes, lemon juice, Italian seasoning, oregano, salt, pepper, and red pepper flakes; bring to a simmer, then reduce heat and cook about 5 minutes until zucchini is tender.
If the sauce is too thin, simmer a little longer to reduce; if too dry, add a splash of broth as needed.
Add the cooked pasta to the skillet and stir to combine evenly with the sauce.
Stir in half of the grated Parmesan until melted, taste, and adjust seasoning (add salt, red pepper flakes, or a pinch of sugar to balance acidity) as desired.
Garnish with the remaining Parmesan and sliced fresh basil, then serve immediately.
