Preheat oven to 450°. Place a large jelly-roll pan in oven (leave pan in oven as it preheats). Cut fish into 1½-inch pieces; sprinkle evenly with ¼ teaspoon salt and pepper. Place cornstarch in a shallow dish (such as a pie plate).
Combine 1 tablespoon water and egg in a shallow bowl; stir with a fork. Place panko in a shallow dish. Dredge fish in cornstarch; shake off excess. Dip fish in egg mixture. Dredge in panko. Carefully remove pan from oven; coat with cooking spray. Drizzle oil over pan; tilt pan to coat with oil.
Arrange asparagus on one side of pan and fish on the other. Sprinkle asparagus with 1/8 teaspoon salt. Bake at 450° for 12 minutes or until done, turning fish and stirring asparagus after 8 minutes. Combine mayonnaise, relish, and onions. Arrange 3 ounces fish, ¼ of asparagus, and 1 tablespoon sauce on each of 4 plates.