1 large egg, lightly beaten
2 cups panko
1 tablespoon Old Bay seafood seasoning
2 tablespoons olive oil
1 and 1/2 pounds tilapia fillets, cut into wide strips
1/2 cup mayonnaise
1/4 cup packed fresh flat-leaf parsley leaves, coarsely chopped
1 tablespoon Dijon mustard
1 tablespoon fresh lemon juice, plus lemon wedges for serving
2 teaspoons prepared horseradish sauce
Preheat oven to 475 degrees. Place egg in a shallow bowl; season with salt and pepper. In another bowl, stir together panko, seafood seasoning, and oil. Dip tilapia into egg, letting excess drip off, then coat completely with panko mixture, gently pressing to adhere. Place on a rimmed baking sheet, leaving space between each piece.
Bake (without turning), until lightly browned, 12 to 15 minutes.
Meanwhile, make sauce: In a small bowl, stir together mayonnaise, parsley, mustard, lemon juice, and horseradish sauce; season with salt and pepper. Serve fish with sauce and lemon wedges.