Bring 500ml / 18fl oz (2 1/4 cups) water to a boil in a large, heavy-based pan. Add the dal and cook for about 20 minutes, or until almost cooked but not completely soft.
Drain, reserving about 250ml / 8fl oz (1 cup) of the cooking water.
Add the green chillies, bottle gourd, onions, and tomatoes to the pan and simmer over low heat for about 15 minutes, or until the gourd is half cooked.
Add the ground spices and garlic paste, then season with salt and cook for a further 5 minutes. Add the coconut, chana dal and continue to cook for 2 minutes, adding a little water if necessary. Add the coriander leaves and remove from the heat.