Heat oil in a large pot. Add cumin seeds and wait until they sputter and turn brown, about 30 seconds (careful, they burn quickly!).
Add finely chopped onions and sauté until three-quarters brown, 8-10 mins.
Add the turmeric powder, cumin, and red chili powder. Stir for 30 seconds, then add the finely chopped tomatoes. Let cook for 4-5 minutes, stirring occasionally, until tomatoes are cooked and start to curl.
Add the bottle gourd after draining. Stir for about one minute, then add 2 cups of water. Cover and cook until almost done, approximately 15minutes.
Add to the cooked dal and let boil for another 5-10 minutes so the curry and dal are blended and flavors infused.
Check for salt. Add more hot water if the consistency is too thick. Sprinkle the garam masala on top and stir.
Pour into a deep serving dish. Garnish with coriander.