Lightly saute sliced onions in olive oil. Add spinach. Cook until spinach is wilted. Cool. Place mixture in 9x13" glass baking dish. Top with goat cheese crumbles. Sprinkle with salt and black pepper. (Can be refrigerated at this point for 24 hours.) Preheat oven to 325 degrees F. In a Pyrex pitcher, beat eggs, heavy cream, white pepper, cayenne, nutmeg and mustard until well beaten. Pour over spinach/onion/goat cheese mixture. Top with shredded cheese. Bake until puffy and very lightly browned. Serve hot.