In a small bowl combine 1/2 cup mayonnaise, 2 tbsp dill pickle relish, 1 tsp chopped capers (if using), 1 tsp fresh lemon juice, 1/2 tsp Dijon mustard and 1 tbsp finely chopped parsley. Season with 1/4 tsp salt and 1/4 tsp black pepper. Mix until smooth, taste and adjust seasoning. Chill in refrigerator while preparing the sandwich (at least 10 minutes).

Fish Sandwich

Prepare the fish

Pat fish fillets dry with paper towels. If fillets are large, cut into 4 sandwich-sized portions (about 4-5 oz each). Lightly season both sides with 1 tsp salt, 1/2 tsp black pepper and 1 tsp paprika.

Set up dredging stations

In a shallow bowl whisk together 3 tablespoons all-purpose flour and 2 tablespoons cornstarch. In a second bowl whisk 3/4 cup buttermilk with 1 large egg until combined. In a third shallow dish place 1 cup panko breadcrumbs and sprinkle with 1/2 tsp garlic powder and a pinch of salt and pepper; toss to combine.

Dredge the fish

Lightly coat each seasoned fillet in the flour/cornstarch mixture, shaking off excess. Dip into the buttermilk-egg mixture, letting excess drip off, then press into the panko so the fillet is evenly coated. Place breaded fillets on a tray and let rest 5 minutes to help the coating adhere.

Fry the fish

Heat a large skillet over medium-high heat and add 1 cup vegetable oil (enough to come about 1/4 inch up the pan). Heat until shimmering and a small breadcrumb sizzles immediately (about 350°F / 175°C). Fry fillets in batches without crowding, 3-4 minutes per side, until golden brown and cooked through (internal temperature 145°F / 63°C) and the coating is crisp. Transfer to a wire rack set over a baking sheet and keep warm in a 200°F oven if needed. Drain any excess oil on paper towel only briefly before moving to rack.

Toast the buns

Spread 1/2 tablespoon softened butter on cut sides of each bun. In a separate skillet over medium heat, toast buttered sides until golden, about 1-2 minutes. Alternatively, toast under a broiler for 30-60 seconds, watching closely.

Assemble

Spread 1-2 tablespoons tartar sauce on the bottom half of each toasted bun. Place a lettuce leaf on the sauce, then a fried fish fillet. Top the fish with 2-3 tomato slices and about 2 tablespoons sliced dill pickles. Squeeze a few drops of fresh lemon over the fish, then finish with a small smear of tartar sauce on the top bun and close the sandwich.

Serve

Serve immediately while hot and crisp. Provide extra lemon wedges and tartar sauce on the side.