Mix the dressing ingredients with salt and pepper, then set aside. Heat a large non-stick frying pan, add the duck, skin side down, then fry for 10 mins. (During this time a large amount of fat will come from the skin and it will turn golden, but the flesh shouldn’t cook too much.) Turn the duck breasts over for a second to seal, then take from the pan.
Toss the potatoes in the duck fat and fry, turning frequently, until they are golden. Add the walnuts, stir, then lift everything out of the fat with a slotted spoon and put in a roasting tin. Put the duck on a roasting rack on top. Brush the beetroot with a little of the duck fat and put on a separate dish. This can all be done on the morning if you want to get ahead, but chill the duck and bring back to room temperature before cooking.
To serve: Heat oven to 220C/fan 200C/gas 7. Roas the duck, potatoes and beetroot for 15 mins, then cut the beetroot into wedges. Put the watercress, spring onions and chicory in a bowl and toss with three-quarters of the dressing. Pile onto plates with the sliced oranges and beetroot, then scatter with the potatoes and walnuts. Slice the duck (you can remove the fat first if you like), arrange on top of the salad and drizzle with the remaining dressing. Serve while still warm.