In a small bowl combine 1/2 cup mayonnaise, 1 tablespoon fresh lemon juice, 1 teaspoon chopped capers and 1/2 teaspoon hot sauce (if using). Whisk until smooth and season with a pinch of salt and pepper. Taste and adjust acidity or heat as desired. This will be spread on the toasted buns.

Seafood Salad Filling

Prepare seafood

Gently pat the cooked crab meat and chopped shrimp dry with paper towels to remove excess moisture. Place in a medium mixing bowl and break up any large lumps of crab gently so pieces remain chunky.

Combine filling

Add the finely diced red bell pepper (1/4 cup), 2 tablespoons thinly sliced green onion, 1/4 cup finely diced celery, 2 teaspoons chopped fresh dill, 1 teaspoon lemon zest, 1/2 teaspoon Old Bay seasoning, 1/2 teaspoon kosher salt and 1/4 teaspoon black pepper to the bowl with the seafood. Fold ingredients gently to combine so you don't overwork the crab meat.

Bind with dressing

Add about 3/4 of the prepared lemon-caper mayonnaise to the seafood mixture (reserve the rest for the buns). Fold gently until the seafood is coated but still chunky. Taste and add more mayo, lemon juice or seasoning if needed. The filling should be moist but not runny.

Toppings & Finish

Toast buns

Spread softened butter on cut sides of the 4 potato buns. Heat a skillet over medium heat and toast buns cut-side down until golden and crisp, about 1–2 minutes. Alternatively, broil cut-side up for 45–90 seconds, watching closely.

Assemble sandwiches

Spread reserved lemon-caper mayo evenly on both top and bottom cut sides of each toasted bun. Divide the seafood salad into 4 equal portions and mound onto the bottom halves. Top each with a small handful of baby arugula or bibb lettuce, a sprinkle of extra chopped dill, and a thin lemon slice. Cap with the bun tops.

Serve

Serve immediately while buns are warm. Provide lemon wedges and extra hot sauce on the side if desired. Leftover filling can be chilled up to 2 days; assemble just before serving.