Add a little more Mayo for more. Adjust the amount of Horseradish to "just a hint" or "over the top", if you like.
This even has the fish enjoying it.
Note: Boetje's Mustard, is a Quad City, Illinois thing at Route 80 and the banks of Ol' Miss. They've been bottling a stone ground mustard forever. Folks that move away call back to get some sent to them. Or make a trip back to acquire some, or order from the internet. You can use Stone Ground or Dijon.