Place a large pot over a medium low heat and add the olive oil. Add the celeriac, leek, potato, garlic and onion, and season with salt and pepper. Gently sweat the vegetables until the celeriac starts to soften, around 10 minutes.
Add the vegetable stock and bring the soup up to a boil before reducing to a simmer. Simmer for 20 minutes or until the celeriac is completely tender. Add the parsley and use an immersion blender to puree until smooth, or alternatively blend in batches in a blender. Return to the heat and check the seasoning and adjust as necessary.
Serve with a drizzle of olive oil or a dollop of yoghurt or sour cream.