Pot-roasting is a method of cooking that involves browning a cut of meat and slow-cooking it in liquid. A pot-roasted leg of lamb has soft meat, infused with rich flavor from the broth in which it is cooked. When selecting a leg of lamb to pot roast, ask your butcher for the chump end. The chump end is the top of the leg, which is meatier and more compact. Pot-roasting a leg of lamb requires approximately 2 and 1/2 hours of cooking time.
Combine the red wine, water, white wine vinegar, diced onion and dried oregano in a mixing bowl. Stir the marinade with a wooden spoon to blend.
Place the leg of lamb into a large bowl, and pour the marinade over top.
Cover the lamb with a sheet of plastic wrap and refrigerate for three hours. Turn the leg of lamb once an hour so the meat marinates evenly.
Remove the leg of lamb from the marinade and pat it dry with paper towels. Reserve the marinade.
Pour the vegetable oil into a casserole dish and place over medium-high heat for three minutes. Brown each side of the leg of lamb for three minutes and reduce heat to low. Pour the reserved marinade over the top.
Cover the casserole dish and leave the lamb to cook for two hours or until its internal temperature registers 140 degrees Fahrenheit.
Remove the leg of lamb from the casserole dish and wrap it in aluminum foil. Add the tomato puree, sugar, salt and pepper to the casserole pan, and stir to blend the sauce. Simmer for 10 minutes.
Unwrap the leg of lamb and serve it with the sauce from the casserole dish drizzled over the top.