Heat a large, cast iron skillet over very high heat until it is beyond smoking (it can't be too hot for this dish), at least 10 minutes. Pour 2 tablespoons of butter into each ramekin, 1 ramekin per serving. Set aside and keep warm.
Reserve the remaining butter in the skillet it which you originally melted it. Heat the serving plates in a 250 degrees oven. Thoroughly combine the seasoning mix in a small bowl. Dip each fillet in the reserved butter so that both sides are well coated. Sprinkle the seasoning mix over by hand, allowing a generous coating on each fillet and coating both sides. Pat the seasoning mix onto the fillets.
Cook the fillets one at a time. Place the fillet in the very hot skillet. Pour 1 tablespoon of butter over each fillet CAUTION, 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 the the thickness of the fillet and the heat of the skillet). Turn the fillet over. Pour another tablespoon of butter over the fillet. Cook until the fish is done (about 2 minutes). Repeat with the remaining fillets. Serve piping hot on the heated serving plates with the reserved ramekins of melted butter alongside.