Cut 4 thin slices of the ginger and shred, then finely grate the remainder. Rub ¼ tsp each grated ginger and orange zest into the duck breasts, then grind over plenty of black pepper and set aside. Put both types of potato into a steamer and cook for 15 mins until tender. Transfer to a bowl and add the remaining grated ginger, then blitz with a hand blender until smooth. Cover and keep warm.
Meanwhile, heat the oil in a large non-stick frying pan. Add the onion, garlic and shredded ginger and fry for 5 mins until softened and starting to turn golden. Push the onions to one side, add the duck and cook for 8 mins, turning frequently and stirring the onions occasionally. Lift the duck out of the pan and put on a plate to rest.
Put the broccoli and sprouts into the steamer and cook for 8 mins until tender. Meanwhile, make the sauce. Add 150ml water, ½ tsp orange zest and the bouillon to the onion mixture and bubble to a sauce, then remove from the heat and add the orange juice.
Smear the ginger mash onto two warmed plates and top with the duck. Spoon over the onions and drizzle over the sauce. Scatter with the parsley or coriander, if using, and serve with the broccoli and sprouts.