Heat the olive oil in a large pot over medium heat. Add the diced onion and cook until golden, about 5 minutes.
Mix in the celery, potatoes, vegetable broth, bay leaf, salt, pepper, and cayenne pepper (if using).
Remove the bay leaf and add the chopped dill and parsley. Use an immersion blender to blend the soup until smooth. If using a regular blender, allow the soup to cool a bit and blend in batches.