Preheat the oven to 375°F and coat a baking dish with cooking spray.
Soak your lasagna sheets in hot water. Leave them in the water for 10 minutes while you cook the spinach and make the filling.
Heat a large pan over medium-high heat and melt the butter. Add the spinach and cook until it wilts; about 30 seconds. Season with salt and pepper then let it cool. Squeeze out the excess water from the spinach and chop coarsely.
Place the spinach, ricotta, egg, and Parmesan in a large bowl, season with salt, pepper, and Italian seasoning, and stir to combine.
Spread some marinara sauce on the bottom of the pan and add a layer of the noodles.
Spread 1/3 of the ricotta mixture over the noodles and top with some mozzarella cheese. Then top again with the marinara sauce.
Repeat the layers, ending with the ricotta mixture and mozzarella cheese on top.
Cover the lasagna with aluminum foil and bake for 45 minutes. Uncover the lasagna and bake for an additional 15 minutes until the cheese is melted and browned.