Pat the grouper fillets dry with paper towels. If the fillets are thick, cut into sandwich-sized portions (about 4 portions total). Season both sides with 2 tablespoons lemon juice, 1 tablespoon olive oil, 1 teaspoon kosher salt, and 1 teaspoon freshly ground black pepper. Let rest for 10 minutes at room temperature.

Breading (for pan-fried)

Set up dredging station

In a shallow bowl combine the flour and cornmeal. In a second shallow bowl whisk the egg with the buttermilk until blended. In a third shallow bowl combine the panko breadcrumbs, smoked paprika, garlic powder, onion powder, 1/2 teaspoon black pepper and 3/4 teaspoon kosher salt.

Bread the fillets

Working one piece at a time: dredge a fillet in the flour-cornmeal mixture, shaking off excess; dip into the egg-buttermilk mixture, allowing excess to drip off; press firmly into the panko mixture to coat completely. Place breaded fillets on a wire rack and repeat with remaining fillets.

For Frying / Finishing

Pan-fry the fillets

Heat a large skillet over medium-high heat and add 1/2 cup vegetable oil (enough to cover the bottom about 1/8 - 1/4 inch). When the oil shimmers (about 350°F if using a thermometer) add the breaded fillets, working in a single layer without crowding. Fry until golden brown and cooked through, about 3–4 minutes per side depending on thickness (internal temperature should reach 145°F). Transfer cooked fillets to a paper towel–lined plate or wire rack and season lightly with a pinch of salt. Keep warm in a low oven (200°F) if needed.

Sauce (Tartar-style)

Make the sauce

While the fish rests, combine 1/2 cup mayonnaise, 1 teaspoon Dijon mustard, 1/2 teaspoon lemon zest, 1 teaspoon lemon juice, 1 tablespoon chopped capers, 2 tablespoons finely chopped cornichons or dill pickles, 1 tablespoon chopped fresh parsley, 1/2 teaspoon hot sauce (optional), 1/4 teaspoon salt and 1/4 teaspoon black pepper in a small bowl. Stir until smooth and adjust seasoning to taste. Refrigerate until ready to assemble.

Sandwich Components

Prepare buns and vegetables

Slice the rolls or brioche buns and butter the cut sides with the unsalted butter. Toast the buttered sides in a skillet over medium heat until golden, about 1–2 minutes per side. Meanwhile wash and dry lettuce leaves, and slice the tomato into 4 even rounds.

Assemble sandwiches

Spread about 1 to 2 tablespoons of the tartar-style sauce on both the top and bottom bun. On the bottom bun place a lettuce leaf, a slice of tomato, then one breaded grouper fillet. Top the fillet with 2 pickle slices, more sauce if desired, then place the top bun. Repeat for remaining sandwiches.

Serve

Serve the grouper sandwiches immediately with lemon wedges on the side for squeezing over the fish. Recommend serving with fries or a simple salad.