Peel and slice the sweet potatoes into 1 cm thick slices. If the slices are too big, then cut into half moons. To prevent them from darkening, keep them in a bowl of water, or chop just when the masala is getting ready.

Cut the tomatoes in half. Using a grater, grate all the pulp, leaving the skin intact. Discard the skins.

In a blender, add the peeled and roughly chopped onion, garlic, ginger, coriander powder, red chilli powder, turmeric powder, 3-4 tsp water. Grind this to a fine paste along with grated tomatoes.

In a non-stick pan, heat the oil. Add cumin seeds. Once the seeds splutter, add the asafoetida and the ground masala paste.

Saute this for 5-7 minutes until you get a nice aroma of the spices and the onion and garlic no longer smell raw.

Add the chopped sweet potato pieces to this (draining them from the water, if you have kept them immersed in water). Add 3/4 cup water, 3/4 tsp salt and cover this with a lid. Bring to a simmer. Simmer on a low flame for 8 minutes or so until the sweet potatoes are tender. This also depends on the variety of sweet potato and the thickness of your slices, so check with the edge of a spatula if they are cooked.

Don't allow the curry to turn too dry. If it does turn dry, add little water at a time and bring to a simmer accordingly. Sprinkle crushed kasoori methi (dried fenugreek leaves) in the final stages and give it a stir.

Remove from the pan into a serving bowl, garnish with coriander leaves and serve with rice or roti.