Preheat your oven to 375°F (190°C).
Cook the lasagna noodles according to package instructions. Once cooked, drain and set aside.
In a large pan, heat the olive oil over medium heat. Add in the onion and garlic, cooking until softened.
Add in the oregano, basil, and red pepper flakes. Stir in the crushed tomatoes and let simmer for 10 minutes.
In a separate pot, wilt the spinach over medium heat. Once wilted, remove from heat and let cool.
In a mixing bowl, combine the ricotta cheese, egg, and Parmesan cheese. Set aside.
In a 9x13 inch baking dish, spread a thin layer of the tomato sauce on the bottom.
Layer 4 lasagna noodles on top of the sauce. Spread half of the spinach on top of the noodles.
Spread half of the ricotta cheese mixture on top of the spinach.
Next, add another layer of noodles, and then the remaining spinach and ricotta cheese mixture.
Top this with another layer of noodles and the remaining tomato sauce.
Sprinkle the mozzarella cheese on top of the lasagna.
Cover the dish with foil and bake for 30 minutes.
Remove the foil and bake for an additional 15-20 minutes, or until the cheese is golden brown.
Let the lasagna cool for 10 minutes before serving.