Cut cod into 4 sandwich-sized portions (about 4–5 oz each). Season buttermilk with 1 tsp salt and 1/2 tsp black pepper, then submerge fish pieces in the buttermilk. Refrigerate for 20–30 minutes to tenderize and flavor.

Breading

Prepare dry mix

In a shallow bowl combine flour, cornstarch, paprika, garlic powder, onion powder, celery salt, cayenne, 1 tsp salt, and 1/2 tsp black pepper. In a separate shallow bowl place the panko breadcrumbs.

Dredge fish

Remove fish from buttermilk, letting excess drip off. Dredge each piece first in the flour mixture, shake off excess, then back into the buttermilk briefly, and finally press into the panko breadcrumbs to coat completely. Set breaded pieces on a wire rack for 5 minutes to let the crust adhere.

For frying

Heat oil

Pour about 1 cup vegetable oil into a heavy skillet to reach roughly 1/4 inch depth (enough to shallow-fry). Heat over medium-high until oil reaches 350°F (175°C) or a breadcrumb sizzles immediately on contact.

Fry fish

Carefully place breaded fillets in hot oil without crowding (work in batches if needed). Fry 3–4 minutes per side until golden brown and internal temperature reaches 145°F (63°C). Adjust heat to maintain oil temperature. Transfer cooked fillets to a paper towel-lined plate or wire rack and season lightly with a pinch of salt.

Sauce & assembly

Make sauce

In a small bowl combine mayonnaise, Dijon mustard, lemon juice, chopped capers, hot sauce, and 1/4 tsp kosher salt. Taste and adjust lemon or hot sauce to preference.

Toast buns

Spread butter on cut sides of the buns. Toast in a skillet over medium heat until golden and lightly crisp, about 1–2 minutes per side.

Assemble sandwiches

Spread a generous layer of sauce on both toasted bun halves. Place a handful of shredded lettuce on the bottom bun, add a fried fish fillet, top with tomato slice(s) and pickles, then cover with the top bun. Serve immediately.