Start preparations at least 1 day in advance. Place the lamb in a tray with coarse sea salt, rosemary and thyme and leave it for about 24 hours.
Preheat oven to 200 degrees (95 C). Brush off the salt and rinse the meat well in cold, running water. Place the meat in an ovenproof container just big enough to hold the meat.
Melt the duck or goose fat and cover the meat with the fat. Bake it in the oven for 6-8 hours.
Place the meat on a kitchen towel to dry off some of the fat. Serve with mustard, salad and boiled root vegetables, such as Vanilla Scented Rutabaga.