Pat shrimp dry with a paper towel and place in a food processor fitted with a metal blade. Pulse until shrimp are finely chopped. Transfer shrimp to a medium bowl and add 3 tablespoons of panko, red onion, red pepper, celery, parsley, mayo, mustard, egg, lemon juice, old bay, garlic powder and pepper. Mix gently.
On a small plate, combine 1 cup panko with ¼ teaspoon of salt and pepper. Using a 1/3 cup measuring cup, scoop shrimp mixture and using your hands, for mixture into a 1-inch patty. Place patty on the plate with panko and evenly coat with panko mixture. With a large spatula, carefully transfer shrimp cake to a plate (you may have to re-form the cake a bit). Repeat with remaining mixture. You should end up with 8 patties.
In a large skillet over medium-high heat, add 1 tablespoon of oil. When oil is hot, add 4 patties, evenly spacing them apart. Cook for 3 minutes, carefully flip with the spatula and cook an additional 2-3 minutes. Place cooked cakes on a plate, add remaining tablespoon of oil and repeat the process with the last 4 cakes.
Place 2 shrimp cakes on 4 plates, sprinkle with parsley and serve with lemon wedges and hot sauce (if using).