Trim the lamb and marinate in vinegar, oil, and salt for two hours in the fridge.
Preheat the oven to 180°C.
Sauté sliced onions in 3 tablespoons of oil for 15 minutes until lightly browned.
Blend the chopped onion, garlic, chillies, ginger, and all ground spices into a purée.
Stir the purée and 2 tablespoons of oil into the onions and cook for 5 minutes until thickened.
Transfer the onion mixture to a casserole dish.
Drain the lamb, reserving the marinade, and brown the meat in batches in the frying pan.
Add the browned lamb, reserved marinade, 500ml of water, salt, and bay leaves to the casserole.
Cover with greaseproof paper and a lid, then bake in the oven for 45 minutes.
Stir in the potatoes and continue cooking for one hour until the meat is tender.
Season with salt to taste and serve.