Cook toor dal with bottle gourd pieces, 1/4 cup onion and pinch turmeric - for 3-4 whistles.
Heat a little oil and fry coriander seeds.
Grind them with coconut, red chillies and tamarind to a smooth paste.
Add the paste and salt to the dal.
Add water to make it to required consistency.
Cook till it starts boiling. Continue cooking for few more minutes. Take off heat.
Heat oil separately and fry remaining onions till they turn slightly brownish.
Pour the fried onions over the dal.
Serve hot with rice.