Preheat your oven to 375°F (190°C).
Cook the lasagna noodles according to package instructions, then drain and set aside.
In a large skillet over medium heat, add olive oil. Once heated, add the chopped onion and sauté for about 5 minutes until translucent. Add garlic and sauté for another minute.
Add the fresh spinach to the skillet and cook until wilted, about 2-3 minutes. Remove from heat and let cool slightly.
In a medium mixing bowl, combine the ricotta cheese, egg, oregano, salt, and pepper. Mix well.
In the bottom of the baking dish, spread a thin layer of marinara sauce. Layer 4 lasagna noodles over the sauce.
Spread half of the ricotta mixture over the noodles, followed by half of the spinach mixture, and sprinkle with 1/3 of the mozzarella cheese. Top with another layer of marinara sauce.
Repeat the layers: 4 noodles, remaining ricotta mixture, remaining spinach mixture, another 1/3 of the mozzarella cheese, and more marinara sauce.
Finish with the last layer of noodles, remaining marinara sauce, remaining mozzarella cheese, and sprinkle with Parmesan cheese on top.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes.
Remove the foil and bake for an additional 20 minutes until the cheese is bubbly and golden.
Allow the lasagna to cool for about 10 minutes before slicing and serving.