Heat ghee till hot. Add ginger, chilies, cumin seeds & black mustard seeds. Fry for a few seconds & add asafetida. A few seconds later add the tomatoes & fry for 3 minutes. Add the potatoes, turmeric, coriander & water. Bring to a boil, reduce heat, cover & simmer for 15 minutes. Add the peas, salt & half the chopped coriander. Continue to cook, partially covered, till the potatoes are soft. Garnish with the rest of the coriander & serve.