To prepare a spinach lasagna, you'll first need to prepare the ingredients for the recipe. First, preheat the oven to 450 degrees Fahrenheit (230 degrees Celsius). Then, slice the spinach into 1 inch (2.5 cm) thick slices. Toss the slices with the ricotta cheese and the Parmesan cheese. Spread the cheese mixture on the bottom and sides of the greased 13x9-inch baking dish. Pour the sauce over the top and bake the dish for 35 to 40 minutes.