For the bangers: If you are using the hog casings, rinse in cold water, making sure to run the water through the insides of the casings as well. Soak in several changes of cold water for at least 2 hours.
Toss together the pork butt, fatback, onions, salt, sage, black pepper, white pepper, ginger and nutmeg in a medium bowl. Transfer to the freezer until the meat is very cold and just starts to get crunchy, about 30 minutes. Be careful not to let it completely freeze through.
For the mash: Put the potatoes in a pot and cover with cold water by 2 inches. Bring to a simmer, salt generously and simmer until cooked through, 10 to 12 minutes. Drain the potatoes thoroughly, and then pass through a ricer into a bowl, or mash them using a handheld potato masher. Stir in the milk, cream and butter to combine, and then season with salt and stir in the chives. Be careful not to over-mix, or the potatoes will become gluey. Keep covered in a warm place until ready to serve.
For assembling and cooking the bangers: Set up your meat grinder with a medium die. Grind the pork mixture into another very cold medium bowl. Mix the breadcrumbs, chicken stock and honey in a small bowl and then stir into the sausage mixture thoroughly using your hands or a wooden spoon.
If you are using casings, set up a sausage stuffer. Stuff the casings with the meat mixture and tie knots at both ends. Twist the casings to make 4 to 5-inch sausage links or bangers (do not cut the links). If you are not using casings, divide the sausage mixture into 8 patties.
Preheat the oven to 170 degrees F. If you are making links, heat 4 inches of water in a large saucepan until steaming but not yet boiling. Add the sausage links and gently poach for 5 minutes, and then remove, pat dry and cut into individual links. Heat the oil in a large skillet over medium heat and brown and crisp the sausages on all sides, about 5 minutes for links. For patties, brown on both sides over medium-low heat until cooked through, about 8 minutes. Transfer the sausage to a baking sheet and keep warm in the oven while you make the gravy in the same skillet.
For the gravy: Heat the butter in the sausage skillet over medium heat. Add the onions and cook until very soft and browned, about 15 minutes, and then stir in the flour and garlic and cook for 2 minutes. Whisk in the stock and Worcestershire sauce and gently simmer for 10 minutes. Season with salt and pepper and serve the bangers with the mashed potatoes and gravy.