To heat oven, heat oven to 190C/170C fan/gas 5. Piercing the flesh and skin of the lamb with a sharp knife, create small pockets.
Place a piece of garlic and a handful of rosemary leaves in each pocket. Slice the potatoes thinly and rinse under cold water. Place the fried onions in a large ovenproof or roasting pan.
Add the potatoes and sprinkle with herbs.
Mix the butter and stock. Then add the potatoes and onions. Season the lamb with salt and freshly grated black pepper. Bake for 1 hour 45 minutes.
Let the lamb rest for approximately 15 minutes before carving. Allow the potatoes to rest in the oven until they are ready to be served.