Grind cumin seeds, black pepper, cardamom seeds, cinnamon, black mustard seeds and fenugreek seeds together well in a spice grinder or mortar and pestle.
In a small bowl, combine these ground spices, vinegar, salt, cayenne pepper and brown sugar. Set aside.
Heat half the oil in large frying pan over medium heat. Fry onions, stirring frequently, until they are a rich, dark brown. (Do not undercook the onions, this is important for colour reasons.)
Remove onions with a slotted spoon and puree them in a food processor or blender.
Return onions to pan but turn off the heat. Add 3 tablespoons water (or more if necessary) to the onions and blend until you have a smooth paste. Add the spices and mix in well. This mixture is the vindaloo paste.
Now, put the ginger and garlic in a blender. Add 1 tablespoon water (or more) and blend until you have a smooth paste.
Heat the remaining oil in a wide saucepan over medium heat. When hot, add the ginger-garlic paste. Stir until the paste browns slightly. Add the coriander and turmeric. Stir for a few seconds.
Add the chicken, a little at a time, and brown lightly.
Stir in the vindaloo paste and tomato passata. Stir and bring to a gentle boil.
Cover the saucepan, reduce heat to low, and simmer for about 45 minutes to an hour, or until chicken is cooked.