!For the filling:
Heat a 12-inch cast iron skillet over medium heat. Melt the butter and add the onions, stirring and sautéing until translucent, about 3-4 minutes. Sprinkle in the flour, stirring constantly, for 1-2 minutes more, allowing the flour to cook a bit (this helps eliminate a "floury" taste in your filling, so don't skip this step).
Pour in the chicken broth and the milk, stirring until combined. Turn the heat up to medium-high and bring the mixture to a gentle boil. Reduce the heat to a low and let simmer about 10 minutes, stirring occasionally as the mixture thickens. Add the chicken, peas & carrots, and parsley. Season with salt and pepper to taste.
Preheat oven to 425.
!To Make the Biscuits:
In a mixing bowl, stir together the flour, baking powder, baking soda, salt, and sugar. Cut in the butter using a pastry blender or your fingers until the mixture is crumbly. Stir in the cheddar cheese, followed by the buttermilk, until it all just comes together and forms a thick, sticky dough (be careful not to overwork the dough - this is the key to a fluffy biscuit).
!To Assemble your pot pie:
Drop the biscuit dough in 1/4 cup sized dollops onto the top of the pot pie filling. Sprinkle the biscuits with black pepper. Place the skillet into the preheated oven and bake for 18-20 minutes, until the biscuits are golden. Serve promptly.