Prep and cook the pork knuckles a day or several hours ahead: Peel 3 onions and cut into wedges. Clean and roughly chop 1 bunch of soup greens. Place the pork knuckles in a large pot with the onion wedges, soup greens, 2 bay leaves, 20 peppercorns, and 10 allspice grains. Cover with water until the knuckles are submerged. Bring to a boil over high heat, then skim foam frequently for the first 10 minutes. Reduce heat to low and simmer gently for 60-120 minutes depending on size, until the pork reaches an internal temperature of 63°C (145°F). Taste the broth during cooking and adjust seasoning as needed. Cool the pork and broth completely, then refrigerate. The next day, lift off and discard any solidified fat from the surface.
Prepare the sauerkraut: Peel 2 onions and cut into cubes. Heat 1 tablespoon lard or butter in a large pot over medium heat. Sauté the onions for 3-4 minutes until translucent. Add 1,000 g sauerkraut (pick through and break apart if needed), then deglaze the pot with 125 ml white wine, cooking for 2-3 minutes. Add 150-200 ml of the reserved pork broth. Season with 1 bay leaf, 4 juniper berries, and a pinch of sugar to taste. Reduce heat to medium-low and simmer for 15-20 minutes, stirring occasionally, until the sauerkraut is tender-crisp and relatively dry.
Prepare the pea puree: Cook 300 g frozen peas with roughly 150 ml of the reserved pork broth in a pot over medium heat for 5 minutes until tender. Drain, then puree with a blender or by pressing through a fine sieve. Return to the pot and simmer over medium heat for 3-5 minutes, stirring occasionally, until creamy. Season with a pinch of salt and pepper to taste.
Prepare the toppings: Peel and halve 2 onions, then cut into thin slices. Heat 1 tablespoon lard in a large skillet over medium-high heat. Fry the onion slices for 4-5 minutes, stirring frequently, until golden brown and crispy. Transfer to a plate. In the same skillet, fry 75 g diced ham for 2-3 minutes until crispy.
Gently reheat the pork knuckles in the reserved broth over medium heat for 5-10 minutes until heated through.
Arrange sauerkraut on serving plates, place one reheated pork knuckle on each portion, spoon pea puree alongside, and top with the crispy fried onions and ham.