Cook 12 oz spaghetti in salted water according to package directions and reserve 3/4 cup pasta water before draining.
While pasta cooks, melt 3 Tbsp butter in a large saute pan over medium heat.
Add 1 1/2 lbs Italian sausage removed from casings and cook, crumbling with a wooden spoon, until no longer pink and lightly browned, 10-12 minutes.
Add 1 1/2 cups heavy cream and 1/2 cup reserved pasta water to the sausage.
Bring to a simmer and cook until slightly thickened, 2-3 minutes.
Stir in 6 cups packed baby spinach and 1 1/2 Tbsp fresh lemon zest and cook until spinach wilts, 1-2 minutes.
Add cooked spaghetti to the pan and toss to coat.
Stir in 1/3 cup grated Parmesan cheese and season with salt and pepper to taste.
Add remaining pasta water if needed to thin sauce.
Serve immediately.