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.
While the pasta cooks, 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 often until the sausage is browned and the onion is softened.
Stir in the minced garlic and cook 1 minute until fragrant; if there is excess grease, spoon off a little, keeping enough fat for flavor.
Add the grape tomatoes and chicken broth and sauté about 4 minutes until the tomatoes soften and begin to burst.
Stir in the zucchini, crushed tomatoes, lemon juice, Italian seasoning, oregano, salt, pepper, and red chili flakes; bring to a boil, then reduce heat and simmer about 5 minutes until the zucchini is tender.
If the sauce seems too watery, simmer uncovered a couple minutes to reduce; if too dry, add a splash more broth.
Add the cooked pasta to the skillet and toss to combine so the sauce coats the pasta evenly.
Stir in half the Parmesan cheese, taste and adjust seasoning (add more salt, red pepper flakes, or a pinch of sugar to balance acidity), then remove from heat.
Garnish with the sliced fresh basil and additional Parmesan, then serve immediately.