Place the Cured Pork Knuckle in a large pot and cover with Water. Add a halved Onion, a chopped Carrot, diced Celeriac, the white part of a halved Leek, a couple of Bay Leaf, a few crushed Juniper Berry and a handful of crushed Black Peppercorn. Bring to a gentle simmer, skim any foam, then cover and cook gently until the meat is very tender (about 1.5–2½ hours depending on size). Reserve the cooking liquid.
While the knuckle simmers, make the sauerkraut: in a large skillet render the Smoked Bacon in Lard until the fat is clear. Add sliced onion and sauté until soft, then stir in the Sauerkraut, a splash of Dry White Wine, and Caraway Seeds. Add a little of the reserved braising liquid, lower the heat and simmer, covered, for 20–30 minutes to meld the flavors.
Cook the potatoes: place the Waxy Potatoes in a pot of boiling Water and cook until just tender (about 15–20 minutes). Drain, return to the pot off the heat and toss with a knob of butter; finish with a pinch of Salt and chopped Parsley. Keep warm.
Make the split-pea purée: rinse the Dried Yellow Split Peas and simmer in fresh water or some reserved broth with a chopped small onion, a Bay Leaf and a pinch of Dried Marjoram until completely soft (about 30–45 minutes). Remove the bay leaf, mash or purée until smooth, then stir in Butter and season with more Salt and freshly ground Black Pepper to taste.
When the knuckle is tender, carefully lift it from the braising liquid, pat the skin dry and place on a roasting tray. Preheat the oven to high (220°C/425°F). Brush the skin lightly with German Mustard if you like a tangy glaze, then roast for 15–20 minutes until the skin crisps and browns. (Alternatively, finish under a hot broiler for a few minutes, watching closely.)
Slice or serve the knuckle whole: arrange the meat with a generous mound of sauerkraut, a spoonful of split-pea purée and the buttery potatoes. Spoon a little warm braising liquid over the plate if desired, garnish with extra Parsley and pass additional German Mustard at the table.
