You need to use a good pot to cook this dish. A lid that will close properly and would retain steam. Heat the Pot, Add oil, add cumin seeds, cook 2 mins., add chopped onions cook 5 minutes and add tomatoes cook 5 mins.
Add sliced Karela and Masala. Mix well and cook 5 minutes. Add 1/2 cup water. Cover and reduce heat to simmer. Cook until tender. Add a little water if necessary, check periodically. Don't let it burn.