Grind cumin seeds, black pepper, cardamom seeds, cinnamon, black mustard seeds and fenugreek seeds together in a spice grinder. In a small bowl, combine ground spices, vinegar, salt, cayenne pepper and brown sugar. Set aside.

Heat oil and ghee in large saucepan over medium heat. Fry onions, stirring frequently, until they are a rich, dark brown. Remove onions with a slotted spoon and put them in a blender. Turn off the heat, but do not discard the oil. Add about 3 T water (or more if necessary) to the onions and blend until you have a smooth paste. Add this onion paste to the spices in the bowl. This mixture is the vindaloo paste.

Put the ginger and garlic in a blender. Add about 3 T water and blend until you have a smooth paste.

Heat the remaining oil in the saucepan over medium heat. When hot, add the ginger, garlic paste. Stir until the paste browns slightly. Add the coriander and turmeric. Stir a few seconds. Add the chicken, a little at a time, and brown lightly.

Add the vindaloo paste, tomato sauce and potatoes to the chicken in the saucepan. Stir and bring to a slight boil. Cover the saucepan, reduce heat to low and simmer for about an hour, or until potatoes are tender. Serve over basmati rice.