Put garlic cloves, dried red chillies, fennel seeds and cumin seeds into a mixer jar and grind to a smooth paste with a little water.
Heat oil in a non-stick deep pan, add black gram, curry leaves and Madras onions and sauté for a minute. Add tomato and mix well. Cover and cook for 5-7 minutes.
Roughly mash potatoes on a plate.
Add ginger-garlic paste and salt to the pan and mix well. Add ground paste and mix well. Add 1 cup water and mix well. Add mashed potatoes and mix well. Simmer till the potatoes get heated through.
Transfer into a serving bowl and serve hot garnished with coriander sprig.