Start off by cutting the stew meat into 1/4 inch pieces (you can also coarsely chop the lamb stew meat in a chopper). Heat the olive oil in a pan and sauté the onions until translucent. Add the lamb, garlic, salt, pepper, oregano and chili flakes and sauté till the lamb gets some color on it. Then add the canned tomatoes, diced tomato, sugar and vegetable stock. Mix well, breaking down the whole tomatoes slightly, then bring to a simmer, cover and cook over a low flame for 20 minutes, mixing occasionally. After 20 minutes check the sauce, if there is an excess of liquid, cook on a medium flame uncovered until most of the liquid evaporates. Serve over cooked rigatoni with a sprinkle of shaved parmesan and parsley.