Grind mustard, coriander, fenugreek, peppercorns, bay leaves, salt, cumin, chilli powder, cardamom, cinnamon, clove, turmeric and chillies. Stir in vinegar to make a thick paste.
Mix spice paste with chicken, cover and marinate for 8 to 12 hours.
Melt butter in a large pan or casserole over medium heat. Cook onions and garlic until tender, 5 to 10 minutes. Add chicken and marinade; cook until chicken begins to brown, about 5 minutes. Stir in potatoes, tomatoes and stock. Bring to the boil, cover and reduce temperature to medium-low. Simmer until chicken is cooked through and potatoes are tender, about 30 minutes.
Serve with fresh naan or over steamed rice garnished with coriander.