1. 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.

2. 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 mushrooms, and one-fourth of the ricotta (use your fingers to spread it evenly) , the mozzarella and the Parmesan. Sprinkle some salt and pepper between the layers of tomato sauce and spinach 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.)