Adjust oven rack to middle position and heat oven to 450 degrees. Set wire rack in rimmed baking sheet and spray with vegetable oil spray. Combine ¾ cup flour, Old Bay, salt, and pepper in shallow dish. Whisk remain­ing 6 tablespoons flour, eggs, mayonnaise, and mustard together in second shallow dish. Combine panko and oil in 12-inch skillet and toast over medium-high heat until lightly browned, about 5 minutes. Transfer toasted panko to third shallow dish. Pat haddock dry with paper towels. 

Working with 1 piece at a time, dredge haddock in flour mixture, shaking off excess. Dip in egg mixture, allowing excess to drip back into bowl, then coat with panko, pressing gently to adhere. Transfer to prepared wire rack. 

Bake until crumbs are golden and fish flakes apart when gently prodded with paring knife and registers 140 degrees, 10 to 12 minutes. Season with salt and pepper to taste, and serve with dipping sauce.