Heat the oil in a large soup pot. Add the onion and and sauté over medium heat until golden.
Add the potato, carrots, celery, and celery leaves, optional cauliflower, vegetable broth, and 2 cups water. Bring to a slow boil, then lower the heat and simmer gently, covered, for 15 minutes, or until the vegetables tender but not overcooked.
Season with salt and pepper and remove from the heat. Allow the soup to stand off the heat for two hours or longer to develop flavor.
Just before serving, add the dill and heat the soup through. Add more water if the vegetables seem crowded, then adjust the seasonings. Serve with 3 or 4 matzo balls in each bowl of soup.