Preheat the oven to 200℃ (180℃ fan)/400°F/gas mark 6. Generously butter a large shallow baking dish.
Mix together the garlic and rosemary for the potatoes in a small dish with a little black pepper.
Spread about one-third of the potatoes in the buttered dish then spread half the onions in a layer over the top. Sprinkle with some of the rosemary and garlic mixture. Repeat the layers then finish with a layer of potatoes.
Carefully pour in the stock and dot the top of the potatoes with the remaining butter.
To prepare the lamb
Mix the rosemary and garlic with a little salt and pepper then stir in the oil to form a paste. Spread all over the leg of the lamb
Place the potatoes on the bottom shelf of the oven and place the lamb on the shelf above so that it is directly over the potatoes
Roast the lamb and potatoes for 1 hour 30 minutes. Remove the lamb from the oven and allow to rest in a warm place (loosely cover with foil) for 20 minutes. Check that the potatoes are tender if they turn the oven off and leave the potatoes in the oven with the door slightly open while the lamb rests. Or cook for longer if necessary.
Nutrition information is approximate and is meant as a guideline only. (It does not include seasoning with salt and pepper.)