Preheat oven to 350°F (176°C).
Place 1/2 of the rehydrated potatoes in a 9″ by 13″ greased pan. Next, layer with 1/2 of the cheese, and sprinkle the minced fresh or freeze-dried onion over this layer.
Add the remaining potatoes and drizzle the butter over this layer.
Add the remaining Swiss cheese, then pour the whipping cream over the last layer.
Add salt and pepper to taste. Cover with foil and bake for about 1 hour or until heated through and bubbling.