Preheat oven to 450° F and place two 9-inch cast iron skillets in the oven. Preheating the skillets helps the bread cook evenly, develops a crust, and makes removal of the bread easier.
In a mixing bowl, lightly beat the eggs with a whisk and then add the buttermilk, milk, and butter. Set the wet ingredients aside.
In a large mixing bowl, whisk the cornmeal, flour, baking powder, and salt until evenly mixed. Using a rubber spatula, slowly pour and mix the wet ingredients into the dry and combine.
Carefully remove the preheated cast iron skillets from the oven and coat heavily with pan spray. Equally divide the batter between the two pans and quickly return to the oven. Bake for 25-30 minutes or until done and let cool.
Preheat oven to 375° F. Lightly coat a medium sauté pan with vegetable oil and render the sausage under high heat until it has browned.
Reduce the heat to medium, add the Cajun mirepoix to the pan and sauté until the vegetables are translucent. Be careful not to brown the mirepoix. Remove the mixture from the pan and let cool.
In a large mixing bowl, place the cornbread crumbs, the cooled sausage mixture, and the fat. Mix the ingredients and then add the eggs, stock, thyme, and parsley while gently mixing.
Finally, add the oysters and oyster liquor and mix, making sure to leave the whole oysters intact. Season with salt and pepper to taste. Using pan spray, prepare a 12-inch cast iron skillet and evenly pour the oyster dressing batter into the skillet and bake for 45 minutes or until done.