Melt the butter in a large skillet over medium heat. Add the onions and sauté them for 2 minutes or until they start to just become translucent. Add the chopped mushrooms and continue to cook for 3-4 minutes. Add the garlic, spinach, red pepper flakes, and nutmeg. Sauté for just a minute or so until all the ingredients are well mixed together. Season with salt and pepper.
In a large bowl, combine the ricotta, egg, and parmesan cheese. Add the spinach mixture to the ricotta and stir to combine.
Position a rack in the middle of the oven and preheat the oven to 375 degrees. Spread a little bit of tomato sauce in the bottom of a 9×13 baking pan. Layer with 3 lasagna noodles, 1/2 of the spinach mixture, 1/3 of the tomato sauce, and 1/3 of the cheese. Repeat until all the layers are formed. Ending with noodles, sauce, and cheese. Bake covered for 20 minutes and then uncovered for 10 more minutes. Let stand for 10-15 minutes so that the lasagna noodles can soak up all the extra deliciousness. Cut and serve warm.