Cook the penne pasta in a large pot of salted boiling water until it is al dente. Reserve 2 to 3 cups of the pasta water before draining.
In a large skillet over medium-high heat, brown the sausage, crumbling it as it cooks.
Add the garlic to the skillet with the sausage and cook until it is fragrant, about 1 minute.
Stir in the tomatoes, heavy cream, Parmesan cheese, Italian seasoning, salt, and black pepper. Add the reserved pasta water a little at a time until you reach your desired sauce consistency.
Bring the sauce to a boil, then reduce the heat to medium and cook until the sauce has thickened slightly, about 5 minutes.
Add the cooked pasta and spinach to the sauce and toss to coat.
Garnish with additional grated Parmesan cheese and black pepper (if desired) before serving.
Video
Notes
Use a large, heavy pot to cook evenly and prevent sticking.
Brown the sausage first to build flavor, and reserve 2–3 cups of pasta water to adjust the sauce.
Cook pasta just to al dente, then stir in spinach at the end so it wilts perfectly.
Season in layers as you go, and use freshly grated cheese for the creamiest texture.