1. Preheat the oven to 425°F. Grease a 9-by-13-inch baking dish.
2. In a large bowl, mix the spinach, ricotta, parmesan, egg, salt, and oregano. Arrange one-third of the sweet potatoes in the dish, overlapping the slices slightly. Dollop half of the ricotta mixture on top, then spread evenly. Spread half of the tomato sauce on top, then sprinkle one-third of the mozzarella over the sauce. Repeat the layering once. Arrange the remaining sweet potatoes on top.
3. Cover the dish tightly with foil. (The lasagna can be refrigerated for up to a day at this point.) Bake for 45 minutes, then remove the foil, and sprinkle the remaining mozzarella on top. Bake until a knife easily slides through the center and the cheese on top is melted and golden brown, about 20 minutes. Let rest at least 10 minutes before slicing.
Tip: For freezing and reheating, completely cool the lasagna. Cut into 8 even pieces and freeze in individual portions in freezer-safe resealable plastic bags for up to 1 month. Unwrap and cover with a damp paper towel on a microwave-safe plate. Microwave until hot, about 5 minutes.