Cook the lasagna noodles in a large pot of boiling salted water until just al dente, about 8 minutes. Drain and set aside.
In a pan, heat olive oil over medium heat. Add chopped onion and minced garlic, sauté until fragrant and translucent, about 5 minutes.
Add chopped spinach to the pan and cook until wilted, about 2 minutes. Remove from heat and let cool slightly.
In a mixing bowl, combine ricotta cheese, sautéed spinach mixture, beaten egg, ground nutmeg, and season with salt and pepper. Mix until smooth and well incorporated.
Preheat your oven to 180°C (350°F). Spread a thin layer of marinara sauce in the bottom of a baking dish.
Layer cooked noodles over the sauce, then spread a third of the ricotta mixture evenly on top. Repeat layers—noodles, sauce, ricotta—until all ingredients are used, finishing with a layer of sauce.
Sprinkle shredded mozzarella evenly over the top layer, followed by grated Parmesan cheese.
Cover the dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10 minutes, until the cheese is bubbly and golden brown.
Take the lasagna out of the oven and let it rest for at least 10 minutes. This helps the layers set and makes slicing easier.
Slice into squares, serve hot, and enjoy the cheesy, festive layers with a hint of green from the spinach for a spooky touch.
