Heat two to three tablespoons of oil in a large skillet on medium heat. Sauté the leeks and onions until golden. Add salt and sugar. When very soft, add mustard and peppercorns. Cook for five minutes more.

Meanwhile, tenderize the meat, making it quite thin, and sprinkle with black pepper and garlic powder.

In another, large, pan, sear the meat on both sides, two or three slices at a time, at high heat in one to two tablespoons of oil. Remove from pan and sear the next batch. When all the slices of meat have been seared, put them back in the pan. Cover with the sauce and then cover the pan. Cook on medium-low heat for about one hour. (If you intend to freeze the meat, you can freeze it at this point, once it has cooled.)

Sprinkle with fresh chopped parsley right before serving.