Special equipment: a deep-frying thermometer
1. For the po' boy sauce: Whisk the mayonnaise, parsley, lemon juice, mustard, Worcestershire sauce and hot sauce in a small bowl. Season with salt and pepper. Set aside.
2. For the po' boy filling: Dry the oysters and shrimp thoroughly, and then sprinkle with the Cajun seasoning. Place the flour, eggs and cornmeal into 3 separate shallow dishes. Dredge the oysters and shrimp first in the flour, then in the eggs and finally in the cornmeal. Place on a rimmed baking sheet.
3. Heat 2 inches of oil in a large, heavy pan or Dutch oven over medium heat until a deep-frying thermometer inserted in the oil registers 375 degrees F. Line a rimmed baking sheet with paper towels and set it next to the stove. Fry the oysters and shrimp in batches until golden and crisp, 2 minutes per batch. Sprinkle with salt.
4. To assemble the sandwiches: Open each roll and spread with a generous amount of sauce. Add some pickles, sliced tomatoes and shredded lettuce and top with a mixture of fried oysters and shrimp. Serve with additional hot sauce on the side.