In a small bowl, stir together mayonnaise, pickle relish, lemon juice, Dijon mustard, and black pepper until well combined. Cover and refrigerate until ready to use. This gets better after 15 minutes of resting.

In a shallow dish, whisk together flour, cornmeal, and Old Bay. In another shallow dish, combine buttermilk and hot sauce if using. Pat fish fillets completely dry with paper towels - this is crucial for crispiness.

Working with one piece at a time, dredge fish in the flour-cornmeal mixture, shaking off excess. Dip into buttermilk, letting excess drip off, then return to the flour mixture and press firmly to coat. Place breaded fish on a wire rack and let rest for 10 minutes - this helps the coating adhere.

Pour oil into a large skillet to a depth of about 1 inch. Heat over medium-high until it reaches 350°F on a thermometer. The oil should shimmer and a pinch of flour should sizzle immediately.

Carefully add 2 fillets to the oil, laying them away from you. Fry for 3-4 minutes per side, turning once, until deep golden brown and the internal temperature reaches 145°F. Transfer to a clean wire rack set over a baking sheet. Repeat with remaining fillets, letting the oil return to 350°F between batches.

Spread tartar sauce on both cut sides of each bun. Layer lettuce on the bottom bun, add the hot fish fillet, top with pickle slices, and close with the top bun. Serve immediately while the fish is crisp.