Cover the bottom of a 9-by-13 inch casserole with a layer of sauce. Add a layer of noodles. Cover the noodles with sauce. This way the noodles cook in the oven, saving time and energy. Spread the vegetable mixture over the sauced noodles. Cover with a layer of noodles and another dressing of sauce. Add the spinach to the second layer of sauced noodles. Cover the spinach with the mashed sweet potatoes. Add another layer of the sauce, the final layer of noodles, and a last topping of sauce. Cover the lasagna with thinly sliced roma tomatoes.