Halve the celery stalks and cut into small slices. Peel the potato, cut in half and cut into small cubes.
Bring the celery and potato with the stock and a pinch of salt to the boil in a saucepan, reduce the heat to the lowest level and simmer with the lid on for about 20 minutes. Then drain the vegetables from the cooking water, except for a small amount of residue. It is essential to collect the cooking water - it is a wonderful taste base for the next soup or sauce.
Pound the vegetables with a potato masher and butter until the desired degree of fineness and stir in the finely chopped parsley. Season to taste with the spices.
Possibly fill the puree in a tumbler and serve with any side dish.