In a heavy 2-3 quart sauce pan, heat the ghee over high heat until a drop of water flicked into it splutters instantly. Stir in the ginger and garlic, then add the onions and salt. Lower the heat to moderate and, stirring constantly add the cumin, turmeric and red pepper. Stir in tomatoes and 2 Tblsp. of coriander. Still stirring, cook briskly for 5 minutes, until most of the liquid in the pan evaporates and the mixture is thick enough to draw away from the sides and bottom of the pan in a dense mass. Drop in the peas and potatoes and turn them about with the spoon until they are evenly coated with the tomato mixture.
Stir in the water, bring to a boil over high heat, cover tightly, and reduce the heat to low. Simmer for 10 minutes, or until the peas and potatoes are tender but still intact. Taste for seasonings and serve at once, sprinkled with the remaining Tblsp. coriander and masala.