Scrub potatoes. Place in a medium pot and cover with water. Bring to a boil over high heat; reduce heat to low and cover. Simmer until fork-tender, about 25 minutes. Drain and rinse in several changes of cold water and let stand.
When potatoes are cool enough to handle, peel and cut into 1/2-inch cubes. Place in bowl and toss with 2 tablespoons of the oil.
Heat remaining oil over medium-high heat in a medium saucepan or skillet. When hot, add asafetida, stir and let sizzle for 30 seconds. Add onion and saute until it begins to brown, about 5 minutes. Add the garlic and ginger, stir and cook 3 minutes.
Add the potatoes, turmeric, coriander, cumin and cayenne. Stir well to coat the potatoes and add 1/3 cup cold water.
Add tomatoes, stir, cover and cook 10 minutes, stirring once after 5 minutes.