Place celeriac in a bowl of water with lemon juice added to prevent browning
Heat the olive oil in a large saucepan over a medium heat.
Add the leeks and garlic and cook, stirring often, for 5-7 minutes until leeks soften.
Add the celeriac, potatoes and stock to the pan. Increase heat to medium-high, cover and bring the boil. Reduce heat and simmer, partially covered, for 20-25 minutes until vegetables are tender.
Allow the soup to cool slightly, then place in a blender and puree soup until smooth.
Return to the pan and stir through the chicken, dill and lemon juice.
Heat the soup through, stirring occasionally.
Season with cracked black pepper and serve with a grainy sourdough bread.