Heat 3 tablespoons oil in a frypan and fry the onions until golden brown. Set aside.
Heat a large frypan. Toast the whole cumin seeds until fragrant, then grind in a mortar and pestle or spice grinder.
In a bowl, combine the fried onions, yoghurt, vinegar, salt, sugar, ginger and garlic pastes, cardamom, cinnamon, ground cumin and red chilli powder. Mix well. Add the chicken and let marinate 3 hours.
Heat the rest of the oil in a large frypan and fry 3-4 pieces of chicken at a time over medium heat. Fry until chicken pieces are dark and appear to be slightly burned, about 15 minutes per batch.