1. Bring a large pot of salted water to a boil. Cook the spaghetti until al dente, then drain and set aside, reserving 1/2 cup of the pasta water.

2. In a large skillet over medium heat, cook the Italian sausage, breaking it apart, until fully browned. Remove sausage and set aside.

3. In the same skillet, melt the butter over medium-low heat. Add the garlic and red pepper flakes (if using), sautéing for 1–2 minutes until fragrant.

4. Stir in the heavy cream and let it simmer gently for 3–4 minutes, reducing slightly.

5. Add the mozzarella and Parmesan cheese. Stir until fully melted and the sauce is smooth and creamy.

6. Return the cooked sausage to the skillet and stir in the spinach. Cook until wilted.

7. Toss the cooked spaghetti into the sauce, adding a splash of pasta water if needed to loosen the sauce.

8. Season with salt and pepper to taste.

9. Serve immediately, topped with extra Parmesan and cracked black pepper.