Wash and pat dry the chicken pieces, then place them in a large non-reactive bowl.
In another clean glass bowl, combine the vinegar with all the ingredients needed for the Spice Blend and soak for 15-20 minutes. After soaking, blend the mixture into a fine paste using a blender.
Add the spice paste to the chicken and toss well to evenly coat all the pieces. Allow the chicken to marinate for 4-6 hours.
In a large, deep skillet or wok, heat the oil over medium heat. Add the marinated chicken and cook for 4-5 minutes, stirring often.
Add the chopped onions, tomato paste, salt, and pepper to the chicken. Stir everything well to combine. Finally, add ½ cup of water, cover the skillet, and reduce the heat to low. Let the chicken simmer for 15-20 minutes, or until fully cooked and the oil begins to separate from the curry.
If you prefer a thinner gravy, add water as needed and let it simmer for another 4-5 minutes before serving. I prefer the gravy to be thick and cling to the meat. Serve hot with rice or bread.