Put all of the ingredients into a spice grinder and blitz to a powder. Rub the spice rub all over the duck legs, then cover and refrigerate for 1 hour.
Preheat the oven to 140C/120C fan. Put a wire rack into a deep roasting tray and pour a centimetre of water into it. Put the ginger and spring onions in two piles on the wire rack, then wipe as much as possible of the marinade from the duck legs and place them on top. Seal the oven dish tightly with foil and cook for 4 hours, checking halfway through and topping up water if required.
Make the plum sauce: Stone and roughly chop the plums. Put them into a saucepan with the rice vinegar, brown sugar, soy sauce, mirin, garlic, star anise and 5 spice. Over a medium-low heat, stir until dissolved then keep cooking for around 20 minutes, until the sauce has reduced by half. Keep an eye on it to make sure it doesn’t catch and burn. Take it off the heat and let it cool for 10 minutes. Remove the star anise, then use a stick blender to blitz to a smooth, thick sauce. Cool fully and then keep covered in the fridge until you’re ready to serve.
Finish the duck and serve: After four hours, take the foil off the duck and turn the temperature up to 220C / 200C fan. Roast at the high temperature for 20-30 minutes, turning halfway through. Cook until the duck skin is deep golden brown and crisp, keeping a close eye on it to make sure it doesn’t burn.
Use two forks to shred the duck then serve immediately with steamed Chinese pancakes, homemade plum sauce, thinly sliced cucumber sticks and shredded spring onions.