I found this recipe for Tuna in ” Lobster & Chips” by Trish Hilferty, one of my go to fish cookbooks. She inherited it from Tom Norrington Davis and they both worked at the renowned Eagle in London. You need to use really fesh tuna and waxy potatoes such as Charlotte. The tuna could be replaced with swordfish or other robust meaty fish. The tuna will be medium rare which is the best way to eat it, but if you prefer cook it for a little longer.