No one really knows the origins of the blackened cooking technique. Some say a cook from Louisiana overcooked a steak and hid his error in a deluge of spices, while others credit chef Paul Prudhomme and his blackened redfish recipe for spreading its popularity. No matter its history, "blackening" made its way into the culinary lexicon, right along with Cajun-style. More than a mix of spices, the blackening technique creates a crust on the outside of meats and fish that straddles the line between well done and burnt. You can adapt the blackening technique to most fish if they are thin, like flounder.
Heat the oven to 500 degrees Fahrenheit and place a cast-iron skillet on the middle rack. Heat the skillet until it turns white hot, about 30 minutes.
Melt 2 or 3 tablespoons of whole, unsalted butter for each flounder filet in a saucepan over low heat. Turn the heat off as soon as the butter liquefies -- well before it separates and clarifies -- and pour it in a shallow dish.
Mix your choice of the spices commonly found in Cajun cooking in a mixing bowl using a whisk to use for your blackening seasoning. Blackening mixes usually include salt, dried oregano, garlic powder, chili powder, dried thyme, dried basil, paprika, red pepper flakes, onion powder and ground black and white pepper. However, you can use any combination of dried spices and still get a crust with the blackening technique. Taste the spices after mixing and adjust the balance as needed.
Pour the blackening spices in a shallow dish. Place the dishes of spices and melted butter near the oven so that you don't have to take the skillet out of the oven to add butter to it later.
Coat the flounder filets in the melted butter on both sides. Lay each filet flat in the blackening spice and tamp them down lightly with your fingers so the spices stick thickly. Repeat on the other side and place the filets on a plate.
Open the oven and pull the rack with the skillet on it out of the oven so you can access it. Pick up each filet one at a time with a metal fish spatula and lay them in the cast-iron skillet. Push the rack back in the oven immediately and close the oven door.
Open the oven door after 1 1/2 minutes and pull out the rack. Pour a tablespoon or two of butter on each filet, using a spoon or a ladle. Keep your face as far from the skillet as you can when adding the butter.
Turn the filets over with the spatula and close the oven door. Let the filets cook for another 1 1/2 minutes and then pull out the rack. Remove the filets from the skillet with the spatula and place them on a plate.
Push the rack in and then close the oven door. Turn off the oven and allow it to cool completely before removing the skillet.