Step 1

For the tomato sauce: Heat the oil in a medium saucepan over medium heat. Add the onions and 1/2 teaspoon salt and cook, stirring frequently, until softened and translucent, about 10 minutes. Add the garlic and cook for 2 minutes. Add the tomatoes and mash into a chunky puree using a potato masher. Bring to a simmer and partially cover the pan with a lid. Cook, stirring occasionally, until slightly thickened, about 30 minutes. Stir in 1/2 teaspoon salt and pepper to taste. Remove from the heat.

Step 2

For the lasagna: While the sauce is cooking, drain the water from the spinach, place in a clean kitchen towel and wring well to remove any excess water. Break up the spinach into a large bowl. Set aside 1 cup of the mozzarella, then add the remaining mozzarella to the bowl with the spinach. Add the ricotta, Parmesan, nutmeg, eggs, 2 teaspoons salt and 1/2 teaspoon pepper and mix with a wooden spoon or rubber spatula until well combined.

Step 3

Peel the sweet potatoes and trim a small bit from one of the longer ends on each so they will sit flat and not roll. Cut the potatoes lengthwise into 1/4-inch planks.

Step 4

Preheat the oven to 350 degrees F.

Step 5

To assemble the lasagna: Add about 3/4 cup of sauce to the bottom of a deep 9-by-13-inch baking dish. Place a single layer of sweet potato slices along the bottom. Top with one-quarter of the ricotta and spinach mixture and use an offset spatula or the back of a spoon to spread gently into an even layer. Dollop about 3/4 cup of sauce on top of the ricotta mixture. Top with another layer of sweet potato slices facing the opposite direction from the previous layer. Repeat with ricotta mixture and sauce until you have 4 layers, ending with the remaining sauce.

Step 6

Cover the baking dish with foil and bake for 25 minutes. Remove the foil, top with the reserved mozzarella and bake until the lasagna is bubbling, the mozzarella is starting to brown and the sweet potatoes are tender when a paring knife is inserted into the center, an additional 45 to 60 minutes. Let sit for 20 minutes before slicing and serving.