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 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.