Step #1 Dice the onion & cook this until soft in the butter.
Step #2 Add the sliced celery & potato to the pot, add 1 pint water, better, or, just enough water to come up to the level of the vegetables.
Step #3 Bring to the boil, then simmer 10 mins.
Step #4 After 10 mins, add 1 tsp salt, 1/2 tsp pepper & 1 tbsp parsley to the soup.
Step #5 Add 1 pint low fat milk, or, enough to bring the liquid level to what seems reasonable to call a soup!
Simmer a further 5 mins, better, but don't boil it!
Serve.