Line bottom of greased baking dish with one of the pastry crusts. Arrange chicken inside of the baking dish. Sauté the onion in butter until golden and add sausage meat and fry until light brown (just cooked), you can substitute the pork with 2 slices of diced bacon if you want. drain and save the fat. Spread the meat and onion mix over the chicken and then to the saved fat, add flour stir in the chicken broth and the milk, cook this until thickened, then add the seasonings. Pour over chicken and sprinkle mushrooms and peas over the top (if using frozen peas, thaw but do not cook them). Cover with the top pastry and seal edges, slit 3 times in the center and bake at 375 for about 1 hour. The crust should be lightly browned and the inside should be bubbly hot