Heat oil, crackle mustard seeds, saute chopped onions, chopped garlic, green peppercorns, green chilly slit until onions are soft.
Make a paste of flour (maida) with water. Add vinegar, salt and flour paste along with chicken stock/water to the sauted onions.
Let it boil and reduce to half. Add butter and sauce has to become thick.
When the sauce is ready, put in chicken pieces wih remaining green peppercorns and mix well. Add little water and let it cook closed until chicken is done. At this stage, open and let it dry. Add little black pepper powder and mix well. Once its dry serve hot!!