Wash and soak the dal into water for 5-6 hours. Grind the dal into wet grinder with salt and asafoetida without adding water.
Transfer the smooth batter into bowl. Add chopped chilies, coriander, grated ginger, curry leaves and onion. Mix well and beat the batter till to smooth and fluffy.
Heat the enough oil in deep pan. Wet your hand and take a small portion and shape it like medhu vada and press the vada in the centre by thumb.
Place it into hot oil. Fry them till to golden brown. Repeat the same process with remaining batter.
Soak the vadas into water and leave them atleast 30-40 minutes.
Combine salt, sugar and milk into curd and whisk well till to smooth.
Heat the oil in separte pan. Add mustard seeds and curry leaves. Add this tempering into yogurt and mix well.
Squeeze out the water and keep the vadas in curd mixture.
Place the vadas into serving dish and garnish with boondi and coriander leaves. Serve immediately.