Spinach, Mushroom, and Potato Oven Omelet

Heat oven to 325 degrees F and grease an 8-inch or 9-inch casserole dish. If using fresh spinach, wash and pull off any tough stems. Put spinach in pot, cover, and cook over high heat, stirring occasionally until cooked, 3 to 4 minutes. Drain, cool, and chop. Heat oil in large skillet. Add potatoes, sprinkle with 1/2 teaspoon salt, and cook, tossing occasionally, until browned, about 5 minutes. Add onion and cook until browned, about 10 minutes. Remove to platter. Sauté mushrooms in skillet in butter 3 minutes until lightly browned; add to potatoes. In food processor or blender, puree spinach, basil, 2 eggs, and half-and-half. In large bowl, beat remaining 6 eggs. Add spinach puree, yogurt, flour, pepper, nutmeg, and remaining 1 teaspoon salt, and beat until blended. Stir in potatoes, mushrooms, and cheese. Turn into casserole. Place casserole in roasting pan and add hot water to come halfway up side of casserole. Bake 1 to 1 1/4 hours. until tip of knife comes out clean when inserted in center. Can be kept warm in turned-off oven for up to 2 hours. Serve hot, warm, or cold. To reheat: If room temperature, bake 25-30 minutes at 325 degrees F; if cold, bake 45 minutes at 350 degrees F.