Preheat the oven to 375 degrees F and coat a baking dish with cooking spray.
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 a spoon of the marinara sauce on the bottom of the pan and add a layer of the noodles. Think of this step as making a lasagna sandwich, minus the bread!
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 lasagne and bake for an additional 15 minutes until the cheese is melted and browned. This is the "bake it till you make it" phase. Enjoy!