Preheat oven to 325 degrees Fahrenheit.

Pour half the bottle of wine into a resealable bag with peppercorns and meat. Make sure the meat gets covered with wine. Marinate overnight or for three hours in the refrigerator.

Cut celery, carrots, and onion into pieces, place into baking dish with the meat and pour marinade over. Tuck rosemary under the meat and sprinkle meat with black pepper and salt.

Bake, covered, for two and a half to three hours or until tender.

Once cooked, remove the meat from the pan along with one cup reserved gravy. Set rosemary sprig aside for garnish.

Pour all other juices and vegetables from the pan into a small pot and add one cup wine. Simmer uncovered for about 15 minutes on medium heat (liquid level should reduce a bit). Using an immersion blender, blend all ingredients together and then strain through a sieve or strainer to remove any whole pieces.

Meanwhile, preheat another skillet and place margarine in it to melt. Once the margarine has melted, add mushrooms and sauté for two minutes. Pour mushroom and margarine mixture into blended sauce and stir together.

Before serving, dissolve potato starch in cold water; pour into gravy mixture and mix well as you heat it up to serve.

You can serve the sauce poured over the sliced meat or in a separate gravy dish. It is excellent over starchy side dishes or even potato crêpes.