Preheat the broiler. Coat a large baking sheet with cooking spray.
Finely grate 1 teaspoon of zest from the lemon and set aside. Squeeze the juice from the lemon over both sides of the fish. Season both sides of the fish with salt and pepper, to taste, and the garlic powder. Place the fish on the prepared baking sheet and broil until fork-tender, about 3 minutes per side.
Meanwhile, in a medium bowl, combine the mayonnaise, lemon zest, horseradish, chives, and parsley. Serve the fish with the mayonnaise mixture spooned over the top.