Marinate the Fish: Combine milk, cornstarch, eggs, and hot sauce. Let the tilapia loin sit in this mixture for one hour to overnight.
Prepare the Breading: Mix together the cornmeal, cornstarch, and all listed spices for the crust.
Make the Corn Casserole:
Using an immersion blender or blender, pulse the corn a few times so its half creamy, half whole consistency.
Mix in the mashed potatoes, softened cream cheese, butter, half-and-half, and Parmesan cheese. Stir well. Add more half and half if needed.
Pour the corn mixture into a baking pan. Bake at 350°F until golden brown and bubbly, about 30 minutes.
Fry the Fish:
Heat the cooking oil in a cast-iron pan over medium-high heat until the oil starts to snap.
Dredge the marinated fish on both sides with the cornmeal breading.
Place an even layer of fish in the hot oil. Cook for four to five minutes on each side, or until golden brown and floating.
Remove the fish from the pan and place it on a paper towel or wire rack to drain.
Serve: Serve the fried fish with the bubbly mashed corn casserole.