Heat the oil in a large, heavy-bottomed saucepan and add the onion. Cook for 5-8 minutes over medium heat, until soft and translucent.
Add the celeriac and potato and stir until well coated; cook for another minute or two. Pour in the stock and bring to a boil, then turn down and simmer the mixture for 20-25 minutes, until both celeriac and potato are tender.
Add the apple to the soup and cook for a further 5 minutes, until the apple has softened. Remove soup from heat and purée using an immersion blender. Taste, and add salt and pepper as needed. If your apples were very sweet, you may want to add some or all of the lemon juice, as well. Serve hot, drizzled with cream, buttermilk or olive oil.
Recipe and image from: http://www.kitchenist.com/cooking/soup/match-made-celeriac-and-apple-soup/2073
