Remove the bone, most of the fat and all of the gristle from the lamb and cut the meat into 1-inch cubes.
In a large Dutch oven
, combine 2 tablespoons butter, the oil, onion, thyme
, bay leaves, vinegar, stock and salt and pepper, to taste, and bring to a boil over medium-high heat. Add the lamb
cubes and cook, covered, for 1 hour. The meat should be very tender, and there should be very little liquid left in the pan. If there is a lot of liquid in the pan, remove the meat and reduce the liquid over high heat, then return the meat to the pan.
In a small saucepan
the remaining butter and mix in the horseradish and parsley. Cook, stirring, over high heat for 1 minute, then pour the sauce over the meat and toss to coat. Serve immediately.