Add the red lentils, chopped onion, grated celery root and sliced carrots to a pot.
Pour in just enough water to cover them and bring to a boil over medium heat.
Let them boil until tender, about 15 minutes.
Remove from the heat.
Add the rest of the ingredients.
Using an immersion blender, blend everything together until the soup is smooth and creamy. Add more water if needed to reach your preferred consistency.
Notes
Add water at the end to reach the consistency you prefer, and stir in the lemon gradually, tasting as you go. Stores well in the fridge for up to 3 days; thin with a little water when reheating.