For the duck, preheat the oven to 220C/200C Fan/Gas 7.
Dry fry all the spices in a frying pan for 1-2 minutes until the spices start to release their oils. Place in a spice grinder or pestle and mortar and grind to a powder. Sprinkle the spice mix all over the duck. Heat a large frying pan and add the duck skin-side down. Cook slowly until the fat has rendered out.
Place the duck in the oven until cooked through (8-12 minutes depending on the size of the duck breast). Check the duck is cooked by piercing with a skewer. The juices should run clear. Remove from the oven and set aside to rest for 10 minutes before serving.
For the sauce bigarade, simmer the strips of orange and lemon in a little water for 5 minutes to soften. Then turn up the heat and boil to evaporate excess water. Stir in the orange and lemon juice, reserve.
Dissolve the sugar slowly in a small pan and cook carefully, stirring, until lightly caramelised (CAUTION: boiling sugar is extremely hot. Handle very carefully.). Add the vinegar and bubble fiercely for a moment, stirring to make a thick syrup. Stir in the stock and let it bubble up again. Turn down the heat and simmer for 5 minutes. Stir in the reserved zest and juice and reheat.
Remove the pan from the heat and whisk in the butter, a little at a time, to thicken the sauce.
For the cavolo nero, heat a sauté pan and add the butter and 2 tablespoons of water. Once hot add the cavolo nero and cook for 2 minutes, until wilted.
To serve, cut the duck into 8 pieces and place on a serving plate. Add the cavolo nero and pour over the sauce.