Preheat your oven to 375 degrees Fahrenheit.
Sauté the sliced mushrooms in a dry skillet over medium-high heat until browned. Add the olive oil, minced garlic, and spinach, cooking until the greens are wilted. Drain excess liquid.
Mix the ricotta cheese, egg, half the mozzarella, half the Parmesan, salt, pepper, and nutmeg in a bowl until smooth.
Layer a 9x13 inch baking dish starting with marinara sauce, followed by three noodles, one-third of the ricotta mix, one-third of the vegetables, and more sauce. Repeat twice.
Top the casserole with the final three noodles, the remaining marinara sauce, and the rest of the shredded cheeses.
Bake covered with foil for 35 minutes. Remove the foil and bake for an additional 15 minutes until golden brown and bubbling.
Rest the lasagna for 15 minutes before slicing to allow the layers to set perfectly.
