Blackened Fish Sandwiches with Horseradish Tartar Sauce

Make the sandwiches:

Whisk together mayonnaise, sour cream, shallot, parsley, horseradish, lemon juice, cornichons, capers, salt, and pepper in a small bowl. Refrigerate until ready to use.

Stir together paprika, garlic powder, oregano, thyme, salt, and pepper in a small bowl. Sprinkle spice mixture evenly over both sides of fish fillets. Set aside.

Heat a large cast-iron skillet over high. Spread 1 teaspoon butter over cut side of each bun half. Place buns, buttered side down, in hot skillet; cook until well toasted and browned, about 1 minute. Transfer to a work surface. Do not wipe skillet clean.

Add oil and remaining 2 tablespoons butter to skillet; cook over high until butter is melted. Add fish fillets; cook until blackened on bottoms, about 3 minutes. Flip fillets; cook until blackened on other sides and just cooked through, 2 to 3 minutes. Remove from heat, and transfer fish to a plate.

Spread 1 tablespoon tartar sauce over toasted side of each bun half. Arrange fish and lettuce evenly on bottom bun halves. Cover with top bun halves, toasted side down. Serve alongside remaining 1/2 cup tartar sauce at the table.