Peel and dice the celeriac and potatoes. Slice the leeks and chop the garlic.
Heat the oil a large saucepan and add gently sauté the leek, over a low heat for 8 minutes or until softened.
Add the celeriac, herbs and potatoes, and sauté for another 2 minutes, stirring.
Add the stock, bring to the boil, then simmer for about 20 minutes.
Remove and discard the bay leaf, then purée the soup with a hand-held blender until it’s smooth and silky. Add more stock if it’s too thick. Season to taste.
Stir in the apple cider vinegar and serve warm.