Place the lentils in a pan, cover with cold water and bring to the boil. Add the curry powder, cover and cook for 6 minutes over a low heat.
Meanwhile, wash and dry the celery and peel off any tough out strips. Set the leaves aside.
Cut the celery into 5mm slices, add to the lentils, cover and cook for 3-4 minutes.
Wash the celery leaves, shake dry and chop roughly.
Season the lentil soup with salt and pepper. Garnish with yogurt and celery leaves and serve.