Place oil, garlic, salt, oregano, peppercorns, lemon zest and juice in a food processor; process until peppercorns are coarsely ground.
Place lamb and lemon-oregano mixture in a bowl or plastic container; turn lamb to coat thoroughly with marinade. Cover, and refrigerate 2 hours or overnight. Bring to room temperature before cooking.
Heat oven to 400℉. Pour any excess marinade from lamb into a large roasting pan. Add potatoes, tomatoes, onions, and carrots; toss to coat. Place lamb on top of vegetables. Roast lamb until browned, 30-40 minutes. Reduce heat to 350℉ and continue cooking until a meat thermometer reaches 125℉, another 30-40 minutes. Turn off oven; let meat rest in oven 30 minutes. Slice lamb and serve with roasting pan vegetables and juices.
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THIS MEAL-IN-ONE-PAN recipe was inspired by the produce at the Bergerac bio-marché. Feel free to be moved by the vegetables at your own local greenmarket (beets or fennel bulb also work well). Gigot d’agneau is now our secret party weapon, perfect for evenings when we’d rather be drinking wine with our guests than cooking. Its deliciousness is derived from the lamb, vegetables, and lemon-oregano marinade roasting together very slowly; most of the work is done before the guests arrive.