If you're using dried pasta, bring a large pot of salted water to a boil. If you're using fresh pasta sheets, cut them into long wide noodles approximately 3 inches by 13 inches, or a size that will fit into your lasagna dish. Cook the noodles (6 at a time for dried noodles) until they are tender but still underdone (they will finish cooking as the lasagna bakes); fresh pasta will take only a minute. Drain and then lay the noodles flat on a towel so they won't stick.

Heat the oven to 400 degrees. Grease a rectangular baking dish with the olive oil, add a large dollop of tomato sauce and spread it around. Put a layer of noodles (use four) in the dish, trimming any overhanging edges; top with a layer of tomato sauce, one-third of the spinach, and one-fourth of the ricotta (use your fingers to spread it evenly), the mozzarella and the Parmesan. Season with salt and pepper if desired.

Repeat the layers twice, and top with the remaining noodles, tomato sauce, ricotta, mozzarella and Parmesan; the top should be covered with cheese; add more ricotta and Parmesan as needed. (The lasagna may be made ahead to this point, wrapped tightly and refrigerated for up to a day or frozen. Bring to room temperature before proceeding.)

Bake until the lasagna is bubbling and the cheese is melted and lightly browned on top, about 30 minutes. Remove from the oven and let rest a few minutes before serving, or cool completely, cover well, and refrigerate for up to 3 days, or freeze.

For Lactose Intolerant:
Boil pasta. Coat with tomato sauce and top with extra spinach.

For Gluten Free:
Boil gluten free pasta. Coat with tomato sauce and top with extra spinach and cheese.