Cook spaghetti in salted water according to package directions and reserve 3/4 cup pasta water before draining.
Melt butter in a large saute pan over medium heat.
Add sausage and cook, stirring and crumbling, until browned and no longer pink, about 10-12 minutes.
Add heavy cream and 1/2 cup reserved pasta water to the pan and simmer for 2-3 minutes until slightly thickened.
Stir in spinach and lemon zest and cook until spinach wilts, about 1-2 minutes.
Add cooked spaghetti to the pan and toss to coat with sauce.
Stir in Parmesan cheese and season with salt and pepper to taste.
Add remaining pasta water as needed if sauce is too thick.
Serve immediately.