1. Bring a large pot of salted water to a boil and cook the spaghetti until al dente according to package directions. Reserve about a cup of pasta water, then drain and set aside.

2. In a large skillet over medium heat, add olive oil. Remove the sausage from its casing and cook, breaking it into crumbles, until browned and cooked through. Transfer to a plate.

3. In the same skillet, reduce heat to medium-low and melt the butter. Stir in minced garlic and cook until fragrant, about 1 minute.

4. Add the cooked spaghetti to the garlic butter, tossing to coat. Return sausage to the skillet, add spinach, and mix until wilted.

5. Sprinkle in Parmesan and mozzarella cheeses, stirring gently until melted. Add pasta water as needed to loosen the sauce.

6. Season with salt and pepper to taste. Serve immediately with extra Parmesan if desired.