I think you're right that MSG will probably do the trick! Adding shiso, miso, mentaiko or whatever should make that lack less prominent. I'm hoping! In Japan, I'd just buy mentaiko mayonnaise, but that doesn't seem to be available in the UK. It seems a shame to add fresh mentaiko, which I can buy, to not-exactly-fresh mayonnaise. Thanks for the recipe, Hiroyuki, I'll try adding some cider vinegar next time. Who knows what will happen!