Peel and lightly mash sweet potatoes and potatoes. Combine together mashed sweet potatoes, potatoes, waterchestnut flour, green chillies, ginger, coriander leaves, crushed cumin, rock salt and lemon juice.
Mix well and divide into twelve equal portions. With wet hands, shape into balls and flatten slightly. Heat sufficient oil in a pan and shallow fry the tikkis until light golden brown on both sides.
Drain onto an absorbent paper. Serve hot with khajur ki chutney.