Preheat oven to 350°F. In a large pot of boiling, salted water, add sliced potatoes and cook for 5 minutes. Drain well and set aside to cool.
Meanwhile, in a large skillet, add the sausage, onions, garlic & mushrooms - cook over medium heat, stirring to crumble, until the sausage begins to brown. Drain off the fat. Stir in flour and nutmeg - season with salt and ground white pepper. Stir in the milk and cook, stirring, until thickened and bubbly. Remove from the heat and set aside.
In a medium bowl, combined ricotta cheese, spinach, Parmesan cheese, egg and crushed red pepper - season lightly with salt and ground white pepper.
In a 3-quart rectangular baking dish coated with cooking spray, arrange half of the sliced potatoes over the bottom. Spread half of the ricotta mixture and half of the meat mixture on top. Sprinkle with half of the mozzarella cheese. Repeat with remaining potatoes, spinach mixture and meat mixture. Reserve the remaining mozzarella.
Cover the dish and bake for 25 minutes. Uncover and sprinkle with remaining mozzarella cheese - place back into the oven and continue to bake until the cheese has melted and the mixture is bubbly around the edges. Remove and let sit for 10 minutes before serving.