Marinate the chicken pieces in the black pepper, turmeric powder, white vinegar and salt for about 30 mins
Crush together the pearl onions, garlic, ginger and fennel seeds in a mortar and pestle to a coarse paste
Heat the coconut oil in a kadhai or heavy bottomed pan. Add the chopped onions, curry leaves and green chillies and saute till the onions start to turn brown.
Add the turmeric powder, coriander poweder, cumin powder, red chillie powder and garam masala. Saute for a couple of minutes more.
Add the crushed paste of pearl onions and garlic and saute till the raw smell of garlic disappears.
Add the chopped tomatoes and cook till tomatoes soften.
Now add the marinated chicken pieces and the cubed potatoes along with a little (about 1/3 cup) water. Add salt and cover and cook over medium high heat till the chicken cooks through.
Remove lid and cook over medium high heat till the curry sauce thickens a little.
Now add the coconut milk and mix well. Cook for 2-3 minutes more and remove from heat.
Heat the coconut oil for tempering and add the pearli onions and curry leaves to it. Once the onions start crisping up a little, pour this over the chicken curry.
Close the curry with a lid, immediately, to trap in the flavors of the tempering.
Serve hot with rotis or palappams or idiappams.