Add curry powder, cumin, mustard seed, coriander and cook for 1 minute, stirring often.
Stir in lentils, sweet potatoes, broth, tomatoes. Bring to a boil and let it simmer for 25 minutes, covered, or until the lentils and sweet potatoes are tender.
Stir in chickpeas and cook until they are heated through, about 2 minutes.