For some reason he decorated the edges with mustard and ketchup.
*UPDATE
Mr. Crompton has spoken…
"Because I didn't want to put it on the pie as that would overpower it, and they had just so happened to stack it above the mash, so that rules out me putting it in between the mash and the pie, so the only option to allow full flexibility in terms of "With this forkfull I will have a bit of mustard, and a dash of ketchup" they needed to be separated and on the only part of the plate available."