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 ramekins; set aside and keep warm. Reserve the remaining butter in its skillet. Heat the serving plates in a 250°F 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 teaspoon 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. Serve each fillet while piping hot.

To serve, place one fillet and a ramekin of butter on each heated serving plate.