Heat oven to 160C/140C fan/gas 3. Place the duck legs on a wire rack, skin-side up, with a tray underneath to catch the fat. Put in the oven and roast for 1 hr. Meanwhile, to make the glaze, pour the soy sauce, maple syrup, honey and juice into a saucepan and bring up to the boil. Add the cinnamon stick and ginger and simmer until the glaze is sticky, then leave to cool.
After 1 hr, turn the oven up to 180C/160C fan/gas 4. Brush the glaze generously over the duck and continue to cook for 30 mins, brushing with glaze every 5 mins or so, until the duck is a deep mahogany with a caramelised crust. Remove from the oven and leave to rest.
While the duck is roasting, make the dressing by whisking together all the ingredients, except the oil, in a pan. Simmer until reduced by half, then whisk in the oil. If you like, you can char the orange segments. Lay them on a metal baking tray and caramelise them with a blowtorch (you only need to colour one side). Don’t blowtorch on parchment or a non-heat resistant plate.
To serve, tip the watercress and orange segments into a large bowl and toss in half the dressing. Divide the salad between the plates, place a crispy duck leg on top and drizzle with a little extra dressing.