Combine celery, potato, onion, and butter in a medium saucepan over medium heat; season with kosher salt. Cook, stirring, until onion is tender, 8–10 minutes. Add broth; simmer until potatoes are tender, 8–10 minutes. Purée in a blender with dill; strain. Stir in cream. Serve soup topped with celery leaves, sea salt, and oil.