1. Preheat Roaster Oven to 375°F.
2. Over medium heat, saute the onions and mushrooms in the butter for about 5 minutes.
3. Add the spinach, Worcestershire sauce, cream cheese, salt, pepper, and nutmeg. Cook until the cream cheese has melted.
4. Spoon into a greased 6-cup casserole dish and bake for 15 to 20 minutes. This can be prepared one day ahead of time and refrigerated. If it has been prepared ahead of time, place in roaster oven on insert rack. Bake for at least 30 minutes.