Soak the potatoes in water for 15 minutes. Drain and pat dry. Heat oil in saucepan. Add ginger, garlic, cumin and chiles. Cook for about 10 seconds. Add potatoes and turmeric. Fry for 5 minutes. Add 1/2 cup boiling water and bring to a boil. Reduce heat, cover and simmer over low for 10 minutes. Add peas and tomatoes. Continue to cook for 4 minutes. Season with salt and pepper. Garnish with cilantro and lemon juice. Serve over rice.