Preheat oven to 180 degrees C. Fill an 20cm x 20cm baking dish with 3cm of water.
Beat the egg, eggnog and sugar together in a bowl. Pour into two small baking dishes or ramekins. Sprinkle tops with nutmeg. Place the ramekins into the dish with the water. Add more water if necessary to reach halfway up the sides of the ramekins.
Bake in preheated oven until tops are set, 35 to 45 minutes. Cool before serving.