Peel and cube potatoes. Chop onion and celery. Saute' for 3 to 5 minutes in hot vegetable oil. In a large pot, add all of the vegetables and cover with with just enough boiling water to cover. Place bay leaf and salt in pot and boil vegetables until tender. Drain vegetables and reserve liquid. Mash vegetables into vegetable stock; add butter. Thin soup with milk as desired; heat until warm. (DO NOT boil).