Heat olive oil in a large soup pot over medium heat. Add onion and saute for 1-2 minutes. Add the celery, potatoes, stock, bay leaf and salt and pepper.
Increase heat to high to bring to a boil then lower to medium-low and let simmer for 25-30 minutes or until the celery and potatoes are soft. Remove from heat. Let cool slightly before blending. Remove bay leaf.
Transfer cooked vegetables to a blender or food processor using a slotted spoon. Add about 1 1/4 cups of broth, reserving the rest in case more is needed. Blend the soup in a blender or food processor in batches. If no more liquid is needed to reach the desired consistency, discard. Return the pureed soup to the pot. Heat over medium heat and taste and adjust seasonings if needed. Serve with a sprinkle of parsley or a drizzle of Greek yogurt (if not vegan).