Whole cumin, coriander, and mustard seeds lend authentic Indian flavor. If you prefer not to eat them, leave them in the bottom of the pot when serving.
Heat oil in a medium saucepan over medium-high heat. Add cumin seeds, coriander seeds, and mustard seeds. Cook, shaking pan often, until seeds are fragrant and begin to pop, about 30 seconds. Add turmeric, curry leaves, ginger, garlic, and chile; cook, stirring, 1 minute.
Stir in tomatoes, stock, and salt; bring to a boil. Add sweet potato; return to a boil. Reduce heat; simmer until sweet potato is tender, about 10 minutes. Serve with lime wedges and cilantro.