In a large, heavy-bottomed Dutch oven, cook the diced bacon over medium heat until it starts to render its fat and turns slightly crispy, about 5 minutes.

Add the bone-in pork shoulder, rinsed split peas, and bay leaves to the pot, then pour in the water. Bring the mixture to a boil over high heat, skimming off any foam that rises to the surface.

Reduce the heat to low, cover the pot with a tight-fitting lid, and let it simmer gently for about 45 minutes, stirring occasionally to prevent the peas from sticking to the bottom.

Carefully remove the pork shoulder from the pot. Shred the meat from the bone, discard the bone and excess fat, and return the shredded pork to the soup.

Stir in the diced celeriac, sliced leeks, carrots, and potatoes. Cover and cook on low for another 30 minutes, or until the vegetables are completely tender and the split peas have broken down into a thick puree.

Add the sliced smoked sausage to the soup and simmer for an additional 15 minutes, allowing the sausage to heat through and release its smoky flavor into the broth.

Season the soup generously with salt and freshly cracked black pepper to taste, then stir in the chopped fresh parsley. Serve hot alongside dark rye bread topped with butter or mustard for a truly authentic Dutch experience.