Peel, wash and cut the potatoes into pieces. Wash and peel celery and cut into large cubes. Cook both in boiling salted water for about 20 minutes.
Heat milk and butter in a small pot. Roast pumpkin seeds in a pan without fat. Drain potatoes and celery, add hot milk-butter mixture and mash with a potato masher.
Season with salt, pepper and nutmeg. Wash thyme, shake dry and pluck finely. Sprinkle puree with pumpkin seeds and thyme.