Bring a pot of water to the boil and add the hash browns. Cook for 5 minutes and drain. In a large bowl, mix together the cooked hash browns, mashed potato mix, butter, cream cheese, jack cheese, sour cream, garlic salt, salt and pepper. Stir in the boiling water. Place in a greased 2-quart casserole dish. Bake for 35 to 45 minutes. Sprinkle the onion rings over the casserole and bake for about 5 to 8 minutes longer until onions are golden brown.