Wash both channa dal and toor dal 2 or 3 times. Add dry red chillies to it and soak altogether for 2 hours.
Drain the water and add the soaked dal and dry red chillies to a blender along with cumin seeds, turmeric powder, and curry leaves. Blend it to a coarse paste without adding water.
Make equal-sized patties and steam them for 5-7 minutes. Once done, cool it down and crumble it using an oiled hand or pulse it using a blender for few seconds.
In a frying pan, heat oil. Add mustard seeds, chopped curry leaves, and broken dry red chillies. Allow it to splutter. Add a big pinch of asafoetida.
Add chopped celery to it. After adding salt and water, stir it well.
Cover & cook it for 5 minutes. There should be no trace of water once the celery is cooked.
Add lentil crumble and mix gently until everything incorporated well.
Celery Paruppu Usili ready to serve.