Preheat oven to 350°F. Bring a large pot of water to boil for the pasta. When it comes to boil, add a couple tablespoons of salt. Cook the pasta al dente, following package instructions. Remove pasta from the water and set aside.
Place spinach into a colander over the sink. Press and squeeze out as much liquid as possible. Set aside.
Cook the sausage in oven (about 15 minutes at 350°) or remove from casing and cook in a frying pan on the stovetop. If left whole, slice into discs about ¼-inch thick.
Assemble the lasagne in a glass or ceramic dish, around 8x10-inches. Start with a few tablespoons of sauce, then layer pasta, ricotta, sausage (if using), spinach, mozzarella, and parmesan. Repeat for a total of three layers, ending with cheese on top. Cover tightly with foil and bake for 1 hour, removing foil for the last 10 minutes. Remove from the oven and let sit 10–30 minutes before serving.