1. In a large pan heat the peppercorn, mustard, cumin, coriander, fenugreek, cloves and cinnamon, over a medium-high heat until toasted, 2 mins approx. Allow to cool in a bowl.
2. Grind the roasted spices in a grinder. Place ground down spices to a processor along with paprika, vinegar, turmeric, brown sugar, 1/2 the ginger and 4 garlic cloves.
3. Transfer spices to a bowl and the chicken, cover the chicken with the spices. Cover and leave to marinade in a fringe for 4 hours.
4. Heat the oil in a saucepan, over a medium heat and add the onions. Cook until caramelised before adding in the garlic, ginger and chillies. Cook, while stirring, until soft, approx. 5 mins.
5. Add the chicken to the pan, along with any excess marinade, add in potatoes and 500 ml of water and bring to a boil. Once boiling reduce to medium-low and cook while covered, stirring occasionally until chicken is thoroughly cooked, approx. 25 mins. Remove from heat, season and serve with rice.