Preheat the oven to 375 F. Brush a 9 x 13-inch baking pan lightly with oil or butter. To prepare the pot pie filling: Heat one tablespoon of the butter or oil in a skillet and saute the onions for about 5 minutes. Add the mushrooms for 2-3 minutes then add the remaining vegetables and seasonings all at once.
Cook for about 5 minutes, stirring often, then remove the cooked vegetables to a side dish and set aside. While the vegetables are cooling, add the chicken to flour and toss to coat. Heat the remaining 2 tablespoons butter or oil in the emptied skillet on medium-high heat. Add chicken and any remaining flour and stir until chicken is browned, about 5 minutes. Mix in broth and the cooked vegetables. Bring to boil, scraping up any browned bits. Reduce heat, cover skillet and simmer until chicken is cooked through, about 8 minutes. Whisk white wine and cornstarch together until smooth and add. Increase the heat, stir constantly until the mixture boils for 2 minutes. Adjust salt and pepper and pour the contents of the skillet into the 9 x 13-inch pan.
To prepare the corn meal biscuit crust: Combine the corn meal, flour, sugar, Rumford Baking Powder and salt. Cut the butter into the corn and flour mixture using a pastry cutter or your hands until it resembles coarse crumbs. Make a well in the center and add the milk and egg. Mix briefly with a wooden spoon to moisten, do not over mix. Drop the biscuit mix by the spoonful onto the pot pie filling. Place it evenly over the surface and the edges, leaving some of the filling visible. Bake for about 30 to 40 minutes, until the biscuits are a deep golden yellow and the edges are browned.
* You may use any other combination of vegetables you prefer, such as celery, turnips, corn, okra, peas, etc. The total amount should be about 2 cups. Frozen vegetables can simply be added when the broth is poured into the chicken, no need to pre-cook them.