Heat oven to 190 degrees C. Wrap the beetroot in aluminium foil and bake until easy to pierce with a knife. Let cool, peel and dice.
Heat a splash of olive oil in a saucepan and add the leeks. Saute a few minutes then add the celery root. Cover and simmer on low 10 minutes, stirring occasionally.
Add the beetroot, stock and enough water to cover the vegetables. Bring to a boil and reduce heat to a gentle simmer. Simmer until you can crush the celery root against the side of the saucepan with a wooden spoon.
Remove from the heat and puree with an immersion blender. Add water, if needed, to your desired consistency. Add salt, pepper and lemon juice.
This soup is best made ahead of time so the flavours can mingle. Reheat and serve!