In a large saute pan, over medium heat, add the oil. When the oil is hot, add the onions, carrots and celery. Season with salt and pepper. Saute for 2 minutes. Add the duck to vegetables, season with salt and pepper and saute for 1 minute. Dust the duck with the flour and cook for 2 minutes, stirring constantly. Stir in the tomato paste and cook for 30 seconds. Stir in the red wine and the veal reduction. Add the bay leaves and thyme. Bring the liquid to a boil. Reduce the heat to medium low and simmer for 1 1/2 hours, during the last 30 minutes add the mushrooms. Remove from the heat and cool. Pour the mixture into a deep 9-inch oval dish. Place spoonfuls of the sweet potatoes over the meat mixture. Dot the top of the sweet potatoes with the butter. Place in a preheated 350 degree oven and cook for about 30 minutes or until the potatoes are golden. (**Place a baking sheet under the dish in case the pie starts to bubble over.) Remove from the oven spoon into serving plates and garnish with parsley.