Preheat your oven to 180°C (fan-forced).
Peel the garlic cloves, slicing 3 of them thinly while keeping the rest whole. Remove the rosemary from their sprigs.
Use a sharp knife, making 10-15 small slits in the top of the lamb. Then, insert the garlic slices and rosemary into the slits, and push them in deep to prevent them burning.
Quarter the potatoes and onions, then cut the carrots into large chunks. Place all the vegetables in a roasting pan.
Place the lamb shoulder on top of the vegetables. Squeeze the juice from the lemons over the lamb, letting juice drip onto the vegetables.
Drizzle the lamb with the olive oil and sprinkle with oregano, salt, and pepper. Rub the seasoning over both sides of the lamb.
Season the vegetables with salt and pepper, then scatter the remaining whole garlic cloves and rosemary over the vegetables in the pan.
Pour the chicken stock into the roasting pan, making sure to avoid pouring it directly over the lamb.
Cover the lamb and vegetables with a sheet of wet baking paper, followed by a layer of aluminum foil to seal in moisture.
Roast the lamb in the oven for 3 hours and 45 minutes.
After this time, remove the baking paper and foil, then use a spoon to baste the lamb with the juices inside the pan. Then put it back in the oven, uncovered, for an additional 20 minutes until the lamb is golden brown and crispy. Don't throw away the foil, we will keep this for after.
To check for doneness, use a fork to pull apart the meat. You should also notice that the bones can pretty much slip out with minimal force. If it’s not tender enough, put it back in the oven and continue checking until you are happy.
Once it’s done roasting, remove from the oven and loosely cover the dish with the foil from before. Try and give it a good 15 minutes to let all the juices settle inside the meat.