How to Make It
Preheat the oven to 400°F.
Trim and peel the eggplant, then place them on 2 oiled baking sheets and drizzle with 2 tablespoons of the oil. Sprinkle with 1/2 teaspoon salt and 1/4 teaspoon pepper, then bake, turning the eggplant over once, until tender, about 25 minutes.
In a large heavy pot, heat the remaining 2 tablespoons of the oil over medium high heat until hot. Stir in the onion, garlic, 3/4 teaspoon salt and 1/4 teaspoon pepper and cook, stirring occasionally, until golden, about 6 minutes. Stir in the tomatoes and red pepper flakes and bring to a boil. Cook the sauce, breaking up the tomatoes with a spatula until the sauce is slightly thickened, 12 to 15 minutes. Stir in the spinach and cook until wilted, about 3 minutes. Remove the sauce from the heat and season with salt and pepper to taste.
In a 3-to 3 1/2-quart lasagna dish, make layers of the eggplant and sauce until both are used up. Sprinkle the mozzarella over top and cover with foil. Bake the lasagna until it is bubbling and the cheese is melted, about 45 minutes. Serve.