01 - Preheat the oven to 375°F.
02 - Heat olive oil in a large skillet over medium heat. Add onion and cook until soft, about 4 minutes. Add garlic and sauté 1 minute more.
03 - Add spinach to the skillet. If using fresh, wilt for 2–3 minutes. If using thawed frozen spinach, cook until any excess moisture evaporates. Season with salt, pepper, oregano, and basil. Transfer to a bowl and let cool slightly.
04 - In a large mixing bowl, combine ricotta cheese, eggs, half the Parmesan, and a pinch of salt and pepper. Stir in the cooled spinach mixture.
05 - Spread 1 cup of marinara sauce over the bottom of a 9x13-inch baking dish.
06 - Layer 3–4 noodles over the sauce. Spread one-third of the ricotta-spinach mixture over noodles. Sprinkle with one-third of the mozzarella.
07 - Spoon 1 cup of marinara sauce over the cheese layer. Repeat layering (noodles, ricotta-spinach, mozzarella, sauce) two more times, finishing with noodles and remaining sauce. Top with remaining mozzarella and Parmesan.
08 - Cover the dish with foil (lightly greased to prevent sticking) and bake for 30 minutes.
09 - Remove foil and bake 15–20 minutes more, until the cheese is bubbling and golden.
10 - Let the lasagna rest for 10–15 minutes before slicing and serving.