Fill a large pot with 5 quarts of cold water; wash marrow bones, add to water and bring to a boil.
Meanwhile, remove any sinew from the roast, but leave on the fat.
Place the roast, bay leaves, peppercorns, and juniper berries into the water and let simmer, on low heat for 2 to 2 1/2 hours, skimming fat off the surface of the water.
Cut the onion in half and fry, cut side down, in a dry cast iron skillet until dark brown.
Cut the remaining vegetables into large cubes and add them, along with the onion, to the pot; simmer until the meat is really tender, another hour.
Remove the meat; strain the liquid, then place the meat back in the liquid.
To serve: Slice the meat and arrange on a warmed platter. Pour over a little of the cooking liquid, season with salt and sprinkle with chives. Serve with crispy fried potatoes, chive sauce, and apple-horseradish sauce.