Place the potatoes in a large pot filled with water over heat. Bring to a boil and let it cook for 10 minutes. Drain and allow to steam dry. Season in a bowl with salt, pepper and toss with a drizzle of olive oil until potatoes are evenly coated in it.
In a small bowl combine garlic powder, paprika, oregano, olive oil and season with salt and pepper. Rub seasoning all over the lamb until evenly coated.
Place the lamb leg in a large baking tray, then add the tomatoes, garlic, olives, parboiled potatoes, lemon, and chicken stock. Place in the oven for 1 hour 15 minutes for medium. Cook for a further 15 minutes if you prefer it well done.