Grind the 3 ingredients mentioned in the grinding list to fine powder. keep aside.
Heat oil in a pan. Add the mustard seeds, fennel seeds, cumin seeds, curry leaves and fry for a min. Add onions and fry until onions turn light brown. Add the ginger garlic paste and fry for a min.
Add turmeric powder, chilly powder, grounded masala powder and saute for two mins. Now add the tomatoes and fry till it turns mushy.
Add salt and some water, mix well and bring to boil for 2-3 mins. Now add the chicken, mix well, cover the pan with lid and continue to cook in low medium heat for around 10 mins. Add the Garam masala powder and mix well and cook for around 5 mins.
Add the grounded black peppercorns and chopped coriander leaves, give a quick stir and take off the flame.