Preheat oven to 425°F. Line a cookie sheet with foil and place on the bottom rack of the oven to catch any drips from cooking.In a medium saucepan, melt the butter. Whisk in the flour until combined and clumps start to form. Gradually add in the chicken stock and the milk, whisking to blend, repeating until fully blended and smooth.Continue whisking until somewhat thickened, raising the heat slightly if needed, around 10-15 minutes. This will form a thick sauce.Add leftover meat, vegetables, salt, and peppers. Cook about 5 minutes on medium heat, stirring until heated thoroughly. Add seasoning, salt and pepper, until desired.Pour mixture into prepared pie pastry crust. Roll second piece of pie pastry and top the pie, sealing the edges by pressing with your fingers. Use a knife to cut five 3-inch long vent slits across the top of the pastry. Bake on middle rack, uncovered for 15-20 minutes or until crust is golden and the filling has begun to bubble. Cool for 5-10 minutes before serving.