Cook spaghetti according to package directions. Omit salt. Before draining the pasta, reserve â…“ cup pasta water.While the pasta is cooking, melt butter in a large saute pan over medium heat. Add sausage and cook, stirring and crumbling until no longer pink and lightly browned, about 8-10 minutes.To the pan, add heavy cream and reserved pasta water; bring to a simmer. Add lemon zest. Season with salt and pepper to taste. Stir in spinach and cook just until wilted.Add cooked spaghetti and stir to combine. Sprinkle with freshly grated parmesan cheese. Serve immediately.