Preheat oven to 375 degrees. In large roasting pan drizzle oil and add onions. Place lamb shanks in pan. Cover with tin foil and roast for 20 minutes. Remove from oven and add tomatoes, oregano, a dash of salt and pepper. Toss ingredients to combine. Cover and roast for another 20 minutes. Remove from oven. Add orzo and heated stock. Cover and roast for another 20 minutes. Serve hot.