Wash, peel and cut the potatoes and celery tuber into cubes of about 1.5 cm. Put some water in a saucepan with a steamer. Put the potato and celery cubes in the steamer insert, place in the saucepan and bring to the boil, covered. Steam softly in approx. 15 - 20 minutes. Put the vegetables in a mixing bowl and mash them with the potato masher. Use a whisk to knock in the butter and milk. Finally season with salt, pepper and nutmeg and serve.