Preheat the oven to 500 degrees F.
Put the chopped onions, celery, and carrots in a roasting pan. Sprinkle with a little salt, pepper, and oregano. Add the garlic cloves. Place the lamb in the roasting pan on top of the vegetables. Season the lamb with a little more salt, pepper, and oregano.
Place in the oven for 15 minutes. Add a couple of cups of water to the pan, enough so it doesn't stick.
Lower the temperature to 350 degrees and cook for 90 minutes. Remove the lamb, strain the juice from the roasted vegetables left in the pan, and put into another pan. Take strained vegetable juice, and add just enough cornstarch to thicken. This is the lamb gravy.
Slice the lamb and serve with gravy, roasted potatoes, and choice of vegetable.
This recipe was provided by professional chefs and has been scaled down from a bulk recipe provided by a restaurant. The Food Network Kitchens chefs have not tested this recipe, in the proportions indicated, and therefore, we cannot make any representation as to the results.
Recipe courtesy of Peter Bota, co-owner Metro 29 Diner in Arlington, VA