Blend the spice ingredients until fine. Heat oil in pot and put in the mustard seeds. When the seeds start to crackle, add the sliced onions and fry until onions soften. Add the blended ingredients, cumin and fry until fragrant.
To make the Vindaloo paste, first grind the cumin, peppercorns, mustard seeds, fenugreek leaves, cardamon and cinnamon together until they are a fine powder.
Add the vinegar a little at a time while frying the ground ingredients and stir well. Put in the chicken and fry over high heat for about 5 minutes and the chicken is well-coated with the mixture.
Mix in the vinegar, salt, cayenne and brown sugar; set aside.
Add water, salt to taste and reduce heat and simmer until the chicken is cooked.
Heat 3 tablespoons of the ghee or oil in a large frying pan until melted. Finely chop the onions and fry over a medium heat until brown.
Keeping the oil for later use, put the onions in a blender with 2 or 3 tablespoons of water and blend until a paste consistency. Add the onions to the spices and mix.
Put the ginger and garlic in a blender, add 2 tablespoons water and blend until smooth. Cook the garlic/ ginger paste in the reserved oil, stir until the paste browns.
Add the coriander and turmeric and cook for a few minutes, stirring.
Add the vindaloo paste and tomato sauce. Cover and simmer on low heat for 1 hour.
Peel and chop the potatoes into cubes. Cook them in boiling water until slightly soft, around 10-15 minutes.
Cube the chicken and fry in a little olive oil until white. Add the chicken and potatoes to the sauce for the last 15 minutes or until the chicken is cooked.
Serve with fresh coriander, basmati rice and naan bread.