While the potatoes cook, prepare the rest of the ingredients. Heat the oil in a heavy skillet and add the mustard seed. After a minute or two, add the onion and sauté until softened, then add the ginger and green pepper and sauté 2 minutes. Stir in the turmeric and salt, then add the potatoes. Pour 1 cup of water into the skillet, stir well, and bring to a boil. Add the cliantro and lemon juice and remove from heat.