Peel the onion and finely dice. Peel the ginger and slice it into matchsticks.
Heat a heavy bottom pan flameproof casserole dish until hot. Place the duck legs skin side down and cook for 5 minutes over a medium heat until the skin browns and the fat renders down. Turn the legs over and cook for a further 2 minutes.
Remove the duck legs and set them aside.
Add the onion and ginger to the rendered fat and cook gently until the onions are translucent, about 10 minutes.
Add the flour and cook for a minute or so. Add the stock and bring to a simmer stirring constantly.
Place the duck legs skin side down into the mixture and return to a simmer. Cover and cook gently for an hour.
For the mashed potato:
Peel the potatoes and chop into 2.5cm chunks.
Put the potato into a pan of salted cold water and bring to a boil. Simmer gently for 20 minutes.
Drain the potatoes thoroughly, add a knob of butter, a splash of milk and the mustard. Mash the potatoes to a creamy consistency.
For the kale:
Bring a pan of salted water to a boil and add the kale.
Return to the boil and simmer for 6 minutes.
Drain the kale thoroughly, add a small knob of butter, stir well to coat and season with black pepper to taste.