Remove casing from sausage and cook in a 10-inch skillet with the mushrooms until meat is brown. Drain off fat.
In a large saucepan cook sliced potatoes, covered, in boiling water for 5 minutes (they won't be thoroughly cooked). Drain; set aside.
For filling, stir together egg, ricotta or cottage cheese, and Parmesan or Romano cheese. Stir in spinach.
For sauce, in a medium saucepan cook onion and garlic in hot margarine or butter until onion is tender but not brown. Stir in flour and nutmeg. Add milk all at once. Cook and stir until thickened and bubbly.
Layer half of the potatoes in a greased 2-quart rectangular baking dish. Top with half of the spinach filling. Top with half of the meat mixture, half of the sauce, and half of the mozzarella cheese. Repeat layers, except reserve remaining cheese.
Cover with foil and bake in a 350 degree F. oven about 35 minutes or until potatoes are tender. Uncover; sprinkle remaining cheese atop and bake about 5 minutes more or until cheese is melted. Let stand for 10 minutes. Makes 6 to 8 servings.