Bring a large pot of salted water to a boil and cook the rigatoni according to package directions; 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 and cook 5–6 minutes, breaking up the sausage and stirring often, until the sausage is browned and the onion is softened.
Add the minced garlic during the final minute and cook, stirring constantly, about 1 minute. If there is excess grease you may spoon some off, but a little fat will help flavor the dish.
Stir in the grape tomatoes and chicken broth and sauté about 4 minutes, until the tomatoes begin to soften and burst when pressed with a spoon.
Add the zucchini, crushed tomatoes, lemon juice, Italian seasoning, oregano, salt, pepper, and red pepper flakes. Bring just to a boil, then reduce heat and simmer about 5 minutes, stirring occasionally, until zucchini is tender.
If the sauce seems too watery, simmer a bit longer to reduce; if too thick, add a splash more broth. Taste and adjust seasoning as needed, adding a pinch of sugar if the sauce is too acidic.
Add the cooked pasta to the skillet and stir to combine. Stir in half of the grated Parmesan until melted and mixed through.
Garnish with the remaining Parmesan and sliced fresh basil, then serve immediately.