Lasagna made from scratch can take several hours from start to finish, even without making your own noodles. Pre-cooked noodles, commonly sold as no-boil noodles, drastically reduce the preparation time because you don't have to boil the noodles, wrangle them out of hot water and get them to lay flat without sticking. They typically come in rectangular sheets without curled edges. The noodles are cooked and dried for packaging so they really only require rehydration, which occurs while baking in layers of sauce and cheese.
Spread a thin layer of pasta sauce over the bottom of a large baking pan. You can use meat sauce, a vegetable-based sauce or plain marinara.
Cover the sauce with a single layer of pre-cooked lasagna noodles placed side by side without overlapping. Pre-cooked lasagna noodles generally have perfectly square corners. If your baking pan has rounded corners, cut the corners of the noodles with kitchen shears to fit perfectly in the pan. Similarly, you might need to cut some of the width or length to make a perfect fit. Reserve what's left of a cut piece for piecing together the noodles in upper layers.
Cover the noodles completely with a generous layer of the pasta sauce. The noodles will not cook properly if any parts are left uncovered. If you want to add a filling such as spinach, layer it over this layer of sauce.
Spread your choice of cheese over the sauce layer. You can use shredded mozzarella or a filling made by combining ricotta cheese, egg, mozzarella and Parmesan. Another common cheese combination is a thin layer of cottage cheese covered with a single layer of Provolone cheese slices.
Top the cheese layer with a thin layer of sauce, using just enough to cover the cheese or about the same amount as you used to line the bottom of the pan.
Cover the thin sauce layer with another layer of no-boil lasagna sheets, trimming them as necessary to fit in the pan.
Continue alternating layers until you fill the pan. Add a thick layer of sauce over the noodles, add optional filling, spread a layer of cheese and add a thin layer of sauce before starting the next layer of noodles. You can layer the ingredients above the height of the pan and it will settle as it cooks. The final layer should be noodles, sauce and a topping of only a dry cheese such as mozzarella, Provolone or Parmesan, leaving out wet cheese varieties such as ricotta and cottage cheese.
Set the lasagna pan on a rimmed baking sheet to catch any splatters or spills. Bake the lasagna at 375 degrees Fahrenheit for about 1 hour or until bubbling and heated through.