Blend 5 sliced tomatoes, green chilies and ginger to make a puree. If you prefer mild take the seeds out of green chili before blending.
Slice 1 tomato length wise into 8-10 pieces and set aside.
Heat the oil in a saucepan. Test the heat by adding one cumin seed to the oil. If the cumin seed cracks right away, the oil is ready.
Add the cumin seeds, and asafetida, as seeds crack add tomato puree, coriander, turmeric, sugar and salt. Cover the pot and cook for about 5 minutes.
Add cornstarch mixture and let it cook for 2 more minutes. Cornstarch is added to thicken the gravy. If gravy is too thick add some hot water as needed.
Turn off the heat add tomato slices, garam masala and cilantro, stir and cover the pan for few minutes before serving.