1. Bring the potatoes, water, and salt to a boil, turn down and simmer for about 15 minutes.
2. Melt the butter in a soup pot.
3. Add the onions, celery, and garlic. Saute for about 5 minutes.
4. Add the peas, white pepper, and the potatoes with the water. Add more water if necessary to cover the vegetables. Bring to a boil, turn the heat down and simmer until the peas are tender.
5. Put the soup into a blender or food processor and puree.
6. Return the soup to the soup pot and add the milk or cream.
7. Adjust the seasonings.
8. Reheat before serving.