In a 1 to 1 1/2 quart saucepan, bring the cold water to a boil. Add the quartered potato and boil 10 to 15 minutes or until tender. Drain and mash with a fork. In a small frying pan, melt 2 tb butter. When the foam subsides, add the onions and cook 7 or 8 minutes stirring frequently until they are soft and transparent but not brown. Scrape the onions into a large mixing bowl and add the mashed potato, ground veal, bread crumbs, cream, water, salt, pepper, egg, parsley and cornstarch. Mix well then refrigerate for at least 1 hour. Brush a large wooden pastry board (or another hard, smooth surface) with water and pat or roll out the mixture into a 16"x16" square about 1/8" thick. Your hands or the rolling pin should be moistened with water to prevent the meat mixture from sticking. With a pastry wheel or small, sharp knife, cut the rectangle of meat into 16 squares of 4"x4" each. Put a thin layer of leek slices (about 1 1/2 teaspoons) on each square. With the aid of a knife, or better still, an icing spatula, roll up each square, jelly-roll fashion. Ideally they should now be chilled but may, if necessary, be cooked immediately. Heat 2 tb. Butter in a heavy skillet. When the foam subsides, add the roulades, 4 at a time, turning them gently with a spatula so that they brown on all sides. When they are a rich brown, set them aside on a heated platter in a 200°F oven. Repeat the process, adding 2 tb fresh butter for every 4 roulades. Pour 1/4 cup heavy cream into the empty pan and boil it rapidly for 3 to 5 minutes, until it thickens, meanwhile scraping up the browned bits in the pan with a rubber spatula or wooden spoon. Taste for seasoning adding salt as needed and pour over roulades. If you must, cover the platter with foil and keep warm in a 200°F oven for not more than 15 minutes.