Boil potatoes until soft. Cool, peel, cut into cubes. While potatoes are being prepared: In oil, cook cumin, garlic, ginger, and onions for about 10 minutes. Add salt, chili powder, turmeric, and tomatoes. Cook about 10 minutes. Add peas and water, reduce heat, cover, and cook for about 10-15 minutes. Add potatoes and cook a few more minutes. Original recipe is here.