Preheat oven to 350 degrees. Place ½ of the rehydrated potatoes in a 9″ by 13″ greased pan. Next, layer with ½ of the cheese, sprinkle the minced fresh or freeze-dried onion over this layer. Add the rest of the potatoes and drizzle the butter over this layer. Add the rest of the Swiss cheese and pour the whipping cream over the last layer and salt and pepper to taste. Cover with foil and bake for about one hour, or until heated through and bubbling.