I'll tackle your question not because I know how but rather because I know what I'd try. I presume you mean cooking in the oven, not under the broiler.
I would beat an egg (or eggs), then roll the filets first in flour, then the eggs, and then in the horseradish/Panko mixture.
And then I would bake it, not broil it, perhaps 20-30 minutes at 350F. You should test for doneness during the later part of cooking, and you may need more time than my guesstimate. What you're looking for is the fish flakes with a fork, and has just lost its translucence in the middle. Or you can cook to more doneness if you prefer but too much and it'll dry out.