Note You'll need to order the corned wagyu brisket from your butcher.
The pink colour we associate with corned beef comes from the nitrites used in the brining process, but if you prefer a preservative-free piece of beef, ask your butcher to brine the beef using a saline solution. It might look a little grey but it will taste just as good. If you can't find fresh horseradish, add some Dijon mustard to the béchamel instead. We've used wagyu here, but regular brisket will work as well.