Bring a large pot of salted water to a boil. Cook radiatore according to package directions until al dente. Reserve 1 cup of pasta water, then drain.
While pasta cooks, heat olive oil in a large skillet over medium-high heat. Add sausage and cook, breaking it up with a wooden spoon, until browned and reaches an internal temperature of 160°F (71°C), about 5-7 minutes. Stir in garlic and cook until fragrant, 30 seconds.
Reduce heat to medium. Add roasted red peppers and stir for 1 minute. Pour in heavy cream and bring to a gentle simmer. Stir in Parmesan, red pepper flakes, salt, and pepper. Let cook for 2-3 minutes until slightly thickened.
Add the drained pasta and spinach to the skillet. Toss everything together, adding reserved pasta water a little at a time until you reach a creamy, clinging consistency. Cook for 1-2 minutes until spinach is just wilted.
Taste and adjust seasoning. Serve immediately in a large bowl, topped with extra Parmesan and fresh basil if desired.