Place lamb on a rack in a roasting pan. With the point of a small knife, make several cuts into the meat to allow the marinade to soak in.
Mix tandoori seasoning, garlic and lemon juice in a small bowl to a paste. Stir in yoghurt until smooth. Brush over lamb. Refrigerate overnight (see tips and hints).
Remove lamb from refrigerator and allow to come to room temperature. Preheat oven to 200C. Roast lamb for 1 1/2 hrs.
Meanwhile, peel potatoes and cut into 2cm-pieces. Mix coriander, turmeric and oil in a bowl. Add potato and onion and toss to coat. Transfer to an ovenproof dish and put in the oven with the lamb for the last 30 mins of the lamb’s cooking time, stirring occasionally.
Remove lamb from oven and cover tightly with foil. Stand for 15 mins before slicing. When potatoes are golden, remove and toss through with coconut and coriander. Slice lamb and serve with potatoes and steamed beans.