Soak bengal gram in water for 30 minutes are until they are soaked.
Soak tamarind in 2 cup of water. Once soaked get tamarind juice.
Grind soaked bengal gram, fennel seeds, dry chili to coarse like we do for vadai. Now add finely chopped onion, curry leaves, 1 teaspoon (tsp.) salt and mix well. Now make small Urundai (balls) and keep it aside.
Heat oil in a pan. add mustard seeds, fenugreek seeds. Once the mustard seed stop splattering. Now add onion and sauté until onion become translucent.
Now add chopped tomato and sauté until they become mushy.
Now add tamarind juice, 2 cup water, turmeric powder, chili powder, salt. NOTE: add less salt as we will add salt to Urundai. Cover and cook this for 5 to 6 minutes.
Once when the Kuzhambu boil well. drop the Urundai (balls) in the Kuzhambu without touching each other. cover and cook for 6 to 8 minutes in a simmering (low medium) heat. Once the Urundai (balls) are cooked and right curry Kuzhambu is reached garnish with coriander leaves and serve.