1.Preheat oven to moderate, 180°C (160°C fan-forced).
2.Cut potatoes and onion into 1 cm-thick slices. Place the potato and onion in the base of a lightly oiled baking dish. Pour stock over potatoes.
3.Combine the mustard, garlic, anchovies and rind in a small bowl. Rub lamb all over with mustard mixture. Place lamb on top of potatoes. Bake, uncovered, for about 1 hour 30 minutes or until lamb is cooked as desired.
4.Remove lamb from the dish; stand, covered, for 15 minutes.
5.Meanwhile, increase oven temperature to very hot, 250°C (230°C fan-forced). Bake the potatoes for a further 15 minutes or until browned.
6.Sprinkle the lamb with the herbs; serve with potato mixture and lemon wedges, if desired.