boil sweet potatoes in water to cover till just tender. cool, peel, and cut into 1-inch pieces.
heat oil in large saute pan over high heat. when oil begins to smoke, add mustard seeds, covering pan. when seeds stop sputtering, add chiles. when chiles are toasted, add onion and ginger. saute until onion is lightly browned, then add turmeric and stir.
add sweet potatoes and salt and toss gently to mix. cover and steam over low heat until flavors meld, about 4 minute sprinkle with lemon juice and serve hot.