Preheat the oven to 150°C.
Season the lamb with salt and white pepper.
Melt the fat in a lidded casserole dish and brown the lamb on all sides until golden.
Remove the meat and discard the excess fat from the dish.
Place half the onions in the dish, rest the lamb on top, add the bay leaf, and cover with remaining onions.
Cover the onions with dampened greaseproof paper and a lid, then bake for three hours.
Check the dish periodically to ensure the juices are running and the onions are not sticking.
Transfer the tender lamb to a roasting tin, cover with foil, and return to the oven at 140°C.
Drain the onions and simmer the reserved juices until reduced by half.
Stir the vinegar, anchovy essence, and parsley into the onions.
Slice the lamb and serve on a hot dish alongside the onions.