Combine boiled and mashed potatoes with cooked black chickpeas in a mixing bowl.
Heat oil in a skillet, add cumin seeds and mustard seeds, then sauté onion, green chilies, and ginger-garlic paste.
Mix in turmeric powder, red chili powder, and salt; add potato-chickpea mixture and besan, adjust consistency with water.
Shape mixture into patties, cook in a skillet until golden brown and crispy.
Garnish with fresh coriander leaves and serve hot with preferred chutney or sauce.