Cook the sliced onion in a knob of butter over very low heat until soft. Add the tomato sauce, brown sugar and vinegar and 1 small cup of water and simmer gently until of thick consistency. Set aside. Mix the flour together with the crushed pistachios, herbs & cheese. Slice the chicken length wise into flat thin slices. Spread with the cranberry sauce and place a knob of cheese on top. Roll thee chicken over to create a pocket for the cheese. Pass the chicken through the flour, dip in the beaten egg and coat with the pistachio crumbs. Shallow fry the chicken pieces in hot fat and serve with the tomato relish.