Bring a large pot of salted water to a rolling boil. Add the pasta and cook until 2 minutes shy of al dente. Reserve 1 cup of pasta water before draining. Drain and set aside.

While the pasta cooks, heat olive oil in a large skillet or Dutch oven over medium heat. Add the diced onion and cook until translucent, about 4 minutes.

Add the garlic and summer sausage cubes. Cook, stirring occasionally, until the sausage is browned on all sides and the garlic is fragrant, about 3 minutes.

Toss in the zucchini and cherry tomatoes. Cook for 2 minutes, allowing the tomatoes to soften and release their juices.

Pour in the heavy cream and 1/3 cup of the reserved pasta water. Stir to combine, scraping up any browned bits from the bottom of the pan. Bring to a gentle simmer.

Add the Parmesan cheese and red pepper flakes (if using). Stir until the cheese is melted and the sauce is smooth. Season with salt and pepper.

Add the drained pasta to the skillet. Toss to coat evenly. If the sauce seems too thick, add more pasta water, 1 tablespoon at a time, until it coats the pasta like a light cream sauce.

Remove from heat. Stir in the fresh basil. Serve immediately in warm bowls.