Break the potatoes by hand in bite size pieces and roughly mash them, this should make about 3 cups.
Heat the oil in a saucepan over medium high heat. Oil should be moderately hot, when you add the cumin seeds they should crack right of way. Add cumin seeds, and asafetida, as cumin seeds crack add the tomatoes.
Cook the tomatoes until they are soft and mushy. Add the other spices coriander powder, turmeric, chili powder and ginger, stir fry until tomato start separating from oil. This should take about 2 minutes.
Add potatoes and salt mix them well with spices, add water, as potatoes come to boil cover the pan and lower the heat to medium and let it cook for about ten minutes.
Mix the potatoes and mash some potatoes as mixing the potatoes, this will thickness to gravy. Adjust the water in gravy to your liking; gravy will thicken as it will cool.
Add garam masala and cilantro, and green chilies this is optional. Mix it well turn off the heat and cover the pan.