Finely chop the onion. Peel the parsnips and cut them into small chunks. Squeeze the garlic flesh out of its skins into a small bowl and set aside.
Heat the oil in heavy-based saucepan, then add the onion and gently sauté it for about 5 minutes, until softened. Add the parsnips and beans, then the stock, garlic flesh and bay leaf.
Bring to the boil, then reduce the heat and simmer for about 20 minutes, until the vegetables are tender.
Remove the bay leaf, then add the parsley and purée it with a stick-blender till you get a smooth, silky soup.
Serve the soup with apple cider vinegar or sour cream.