Preheat the oven to 350 °F (175 °C). Pat the lamb dry with paper towels and place it in a large roasting dish. Rub with olive oil and season generously with salt and pepper.

Use a small knife to pierce the top of the lamb in 12 spots. Insert a piece of garlic and a rosemary sprig into each incision.

In a medium bowl, whisk together lemon juice, olive oil, mustard, salt, pepper, oregano, garlic powder, chicken stock paste, and water. Pour the sauce mixture around (not over) the lamb in the roasting dish.

Cover the roasting pan first with parchment paper, then foil. Bake for about 3 hours, or until the lamb is very tender and starts to pull away from the bone.

Increase the oven temperature to 425 °F (220 °C). Uncover the lamb and scatter the potato wedges and smashed garlic cloves around it in the pan. Toss the potatoes to coat them in the sauce and spoon some sauce over the lamb as well.

Roast uncovered for another 45 minutes, or until the potatoes are golden and tender and the lamb is nicely browned. Check after 30 minutes—if the lamb is browning too quickly, cover loosely with foil and continue roasting.

Once cooked, remove from the oven and let the lamb rest for 5–10 minutes. Slice the meat off the bone and arrange on a serving platter.

Serve with roasted lemon potatoes and spoon over the pan juices. Garnish with fresh mint, lemon slices, and tomatoes if using. Pair with tzatziki and Greek salad for a complete meal.