Many people, especially tourists in Thailand, when they look at a restaurant menu and see John Dory Fish and Chips, or just Dory Fish & Chips, priced at say 300-400 Baht per person, jump at that, not understanding, the fish is more than likely freshwater Pangasius (Shark Catfish) from Vietnam. John Dory fish are a deep water ocean fish caught off from Europe and also caught off from New Zealand, the price would be much much higher. Pangasius is about 120 Baht per kilo, that is 2 pounds of fish, keep that in mind.
Common price for Pangasius marketed at Fish & Chips in local pubs will run 150 to 180 Baht per serving, normally 2 fillets, with chips on the side, reasonable for bar grub as they have to store it, cook it, and cook chips, fair enough. To state John Dory or Dory as the fish but serve something else, well, that is common place in touristy areas.
The difference to note is the Pangasius is long and narrow, Dory is shorter, wider and much thicker.
Here are a few photos:
This is John Dory, shorter, wider, and thicker fillets. So next time you are in a high class restaurant, and order Fish & Chips because the menu says Dory and your fillet looks like the one in the photo above the Dory, and you paid top dollar for it, you got robbed, as you would in a high class restaurant anyways.
Both are a firm, tender, flaky, whitefish. In my 10 years living in Thailand, I have never seen true John Dory offered in any store (Villa Market, Tesco, Tops, or even Makro) But I have seen Pangasius offered as Pangasius Dory, which is quite misleading.