Gently pick through lump crab meat to remove any shell pieces; transfer to a large bowl. Add panko breadcrumbs, mayonnaise, egg, Dijon mustard, Worcestershire sauce, Old Bay, chopped parsley, sliced green onion, lemon zest, salt, and pepper. Using a rubber spatula, fold ingredients gently until just combined — avoid breaking up the crab lumps. Chill the mixture for 20 minutes in the refrigerator to help it firm up for shaping.

Shape cakes

Divide the chilled mixture into 4 equal portions (about 4 oz each). Using lightly oiled hands or a 3-inch ring mold, shape each portion into a round patty about 3/4 inch thick. Place shaped patties on a tray and return to the refrigerator for 10 minutes to set.

Cook crab cakes

Heat a large nonstick or cast-iron skillet over medium heat. Add 2 tablespoons vegetable oil and 1 tablespoon butter (if using) and let butter melt and foam. When oil is shimmering (but not smoking), add crab cakes to the skillet leaving space between them. Cook without moving for 3–4 minutes until the bottoms are golden brown and crisp. Carefully flip each cake and add the remaining 1 tablespoon oil if pan looks dry. Cook the other side 3–4 minutes until golden and heated through (internal temp ~145°F / 63°C). Transfer cooked cakes to a paper towel–lined plate and keep warm in a low oven (200°F / 95°C) if needed.

Sandwich Assembly

Toast buns in a dry skillet or under the broiler for 1–2 minutes until lightly golden. Spread about 1 tablespoon tartar sauce on the bottom half of each bun (adjust to taste). Place a lettuce leaf on each sauced bottom bun, then top with a warm crab cake. Add 1–2 slices of tomato and a slice or two of red onion over each crab cake. Squeeze a little lemon juice over the top (about 1/2 teaspoon fresh lemon juice per sandwich) and finish with the top bun.

Serve immediately with extra lemon wedges and additional tartar sauce on the side. Optionally accompany with fries or a simple slaw.