Start by making the Cajun spice blend. In a small bowl, mix the flour, cornstarch, paprika, garlic powder, onion powder, paprika, red pepper, black pepper, and salt. Mix well.
Generously rub olive oil over the flounder fillets, both sides. Coat the fillets with the Cajun flour mix. Press firmly with your hands to make sure that the coating stays on.
Spray your air fryer basket with cooking spray, and then place your coated fillets into the air fryer basket.
Spray VERY GENEROUSLY (the entire coating) with cooking spray, air fry for 5 minutes, flip and SPRAY again with cooking spray, and air fry for another 3 to 5 minutes until golden brown. (Use air fryer setting)
Then spread the butter on the Brioche buns, and place into the air fry for 2 minutes at 350 degrees F, air fryer setting, or until golden brown.
To Assemble: Spread a thin coating of tartar sauce on the top and bottom of the Brioche buns, then place your cooked fish on top, add the pickle slices on top of the fish, and then the top bun.
Plate, serve, and enjoy!