1. Bring a large pot of salted water to a boil. Cook spaghetti until al dente, reserve 1/2 cup of pasta water, then drain.

2. In a large skillet, cook Italian sausage over medium heat until browned and cooked through. Remove and set aside.

3. In the same skillet, melt butter and sauté garlic over medium-low heat until fragrant, about 1 minute.

4. Add fresh spinach and stir until wilted, about 2 minutes.

5. Pour in heavy cream and bring to a gentle simmer. Stir in mozzarella until melted and smooth.

6. Return sausage to the skillet and mix to combine.

7. Add cooked spaghetti and toss everything together, adding pasta water as needed to loosen the sauce.

8. Season with salt, black pepper, and red pepper flakes if using. Serve hot with extra cheese if desired.