I'm just going to post this word for word from the man himself, Paul Prudomme from the cookbook "Paul Prodhomme's Louisiana Kitchen", which is my bible for cajun cooking. As you'll read, one thing he notes is that he doesn't recommend using this on fillets thicker than 3/4 inch. With tuna, that may not be an issue since you probably want it more on the rare side in the middle.
I've made this numerous times, but mainly with snapper. Never tried it with tuna. Be warned though, this kicks up a LOT of smoke. I have to open the doors, run a fan and turn off the smoke alarm!
Paul Prudomme's Blackened Redfish
Makes 6 servings
3/4 pound (3 sticks) unsalted butter, melted in a skillet
Seasoning Mix
1 tablespoon sweet paprika
2 1/2 teaspoons salt
1 teaspoon onion powder
1 teaspoon garlic powder
1 teaspoon cayenne
3/4 teaspoon white pepper
3/4 teaspoon black pepper
1/2 teaspoon dried thyme
1/2 teaspoon dried oregano
6 (8-10-ounce) fish fillets (preferably redfish, pompano or tilefish), cut to about 1/2 inch to 3/4 inch thick.
Heat a large cast-iron skillet over very high heat until it is beyond the smoking stage and you see white ash in the skillet bottom (the skillet cannot be too hot for this dish), at least 10 minutes.
Meanwhile, pour 2 tablespoons melted butter in each of 6 small ramkins; set aside and keep warm. Reserve the remaining butter in its skillet. Heat the serving plates in a 250 degree oven.
Thoroughly combine the seasoning mix ingredients in a small bowl. Dip each fillet in the reserved melted butter so that both sides are well coated; then sprinkle seasoning mix generously and evenly on both sides of the fillets, patting it in by hand. Place in the hot skillet and pour 1 tablespoon melted butter on top of each fillet (be careful, as the butter may flame up). Cook, uncovered, over the same high heat until the underside looks charred, about 2 minutes (the time will vary according to the fillet's thickness and the heat of the skillet). Turn the fish over and again pour 1 teaspoon butter on top; cook until fish is done, about 2 minutes more. Repeat with remaining fillets.
To serve, place one fillet with a ramekin of butter on each heated serving plate.