Preheat oven to 375 degrees Fahrenheit. Heat olive oil in a skillet and brown the mushrooms, then stir in garlic and spinach until wilted.
Mix the ricotta, egg, Parmesan, Italian seasoning, salt, and pepper in a medium bowl until fully combined.
Layer marinara sauce on the bottom of a 9x13 baking dish. Add three noodles, a layer of the ricotta mixture, a layer of the mushroom spinach blend, and a sprinkle of mozzarella.
Repeat the layers until you reach the top, finishing with noodles, the remaining marinara sauce, and a thick layer of mozzarella.
Bake covered with foil for 45 minutes, then remove the foil and bake for an additional 15 minutes until the cheese is bubbling and golden.
