Dice the onion and cook until soft in the butter. Add the diced celery and potato to the pot, add 1 pint water, or, better, just enough water to come up to the level of the vegetables. Bring to the boil, then simmer 10 minutes.
After 10 minutes, add 1 teaspoon salt, 1/2 teaspoon pepper and 1 tablespoon parsley to the soup.
Add 1 pint low fat milk, or, better, enough to bring the liquid level to what seems reasonable to call a soup!