Preheat the oven to 350°F (175°C). Cut the potatoes into small pieces and rinse the lamb. Place both in a roasting pan. Drizzle generously with olive oil, then add salt, pepper, paprika, oregano, allspice, mustard, and garlic powder (use as much of each as you like). Mix well to coat everything evenly. Add enough water to come about halfway up the pan.

Roast for 2 hours, turning the lamb after 1 hour so it browns on both sides. When the potatoes are tender and the lamb is golden, squeeze a little lemon juice over the top.