Pat the fish fillets dry with paper towels. If fillets are large, cut into 4 equal portions (about 4 oz each). Season both sides lightly with 1/2 tsp kosher salt and a pinch of black pepper.

Make blackening seasoning

In a small bowl combine smoked paprika, paprika, onion powder, garlic powder, dried thyme, ground cumin, cayenne pepper, remaining 1/2 tsp kosher salt, and 1/2 tsp black pepper. Mix thoroughly.

Coat fish

Brush both sides of each fillet lightly with the 2 tbs olive oil. Press the blackening seasoning onto both sides of the fish so it forms an even coating. Let rest 5 minutes to adhere.

Cook fish

Heat a large cast-iron or heavy skillet over medium-high heat until very hot (about 3–4 minutes). Add 1 tbs unsalted butter and 1 tsp vegetable oil. When butter is foaming and just starting to brown, add fillets (do not overcrowd; cook in batches if necessary). Cook 3–4 minutes on the first side without moving, until a dark crust develops. Carefully flip and cook 2–3 more minutes until fish is opaque and flakes easily (internal temp ~145°F/63°C). Remove to a plate and tent loosely with foil to keep warm.

Remoulade

While the fish rests, make the remoulade. In a bowl whisk together mayonnaise, Dijon mustard, prepared horseradish, lemon juice, chopped capers, chopped cornichons or pickles, hot sauce, paprika, 1/4 tsp salt, and 1/8 tsp black pepper until smooth. Taste and adjust seasoning or hot sauce as desired. Refrigerate until ready to assemble.

Assembly & Garnish

Toast buns

Split the buns and toast cut sides in the same skillet over medium heat for 1–2 minutes with any remaining butter/oil until golden. Alternatively, toast under a broiler briefly. Remove and spread a thin layer of remoulade on both cut sides.

Assemble sandwiches

On the bottom bun place a lettuce leaf, a slice of tomato, and a few rings of red onion. Top with a blackened fish fillet, 1–2 pickle slices, and a generous dollop (about 1–2 tbs) of remoulade. Finish with the top bun.

Serve

Serve immediately with lemon wedges on the side for squeezing over the fish. Recommended accompaniments: fries, coleslaw, or a simple green salad.