Preheat oven to 230°C.
Rub the lamb with softened butter and place in a roasting tin.
Make deep slits every 4cm in the meat and insert garlic slivers and rosemary sprigs.
Drizzle with 15ml of olive oil and roast for 30 minutes.
Toss sliced potatoes and onions in a bowl and season with salt and pepper.
Remove the meat from the oven and lift it out of the tin.
Layer potatoes and onions into the tin fat and pour over 450ml of stock.
Place the tin on a low shelf and the lamb directly on the rack above it.
Roast for approximately 60 minutes until the meat is cooked to your preference.
Mix the salad leaves with lemon juice and remaining olive oil.
Carve the lamb and serve with the potatoes and fresh salad.