Season the knuckles and dust with flour.
Heat a little oil in the dish, fry the knuckles in batches for approx. 3 mins. each side, remove. Wipe the cooking fat from the pan.
Gently fry the bacon lardons in the roasting pan on a medium heat until they turn crispy. Add the onions, celeriac and sage and sauté for approx. 5 mins. Pour in the beer and stock, bring to the boil, reduce the heat. Return the meat to the roasting pan, cover and braise in the centre of an oven preheated to 160 °C for approx. 1½ hrs. Take out of the oven, remove the lid. Increase the heat to 220°C.
Mix the oil, breadcrumbs and sage and spread over the meat. Gratinate the knuckles in the centre of the preheated oven for approx. 10 mins. until the sage crunch is crispy.