Steam the potatoes in a large covered pan fitted with a steamer basket until they are tender, about 8 minutes.
In a large bowl mash the potatoes with the buttermilk and orange juice. Stir in the zest, salt, and nutmeg. Serve the potatoes topped with the butter, if desired.
Recipe Analysis Note: Ingredients without discrete measurements such as "Salt, to taste" or "Ice cream, optional" are omitted from analysis. This is because amounts can be highly variable and difficult to determine.