Instead of calling this "Tuna & Bok Choy", it might has well be called "[Firm flesh fish] & [Resilient Green Veggie] Pockets." Incidentally, it's also quite easy to customize the sauce that goes into the packet.
I made this with mahi-mahi since it was on sale, and did find baby bok choy. In retrospect, any number of veggies that can stand up to steaming (or grilling) would be fine.
Some may find the horse radish mustard in the sauce a little too assertive, though not only did I use straight up dijon mustard, I also added a little bit of extra kick to it.
Calories per serving is ideal for those looking to either lose weight or keep weight off, and even with a serving of brown rice specked with cubed prosciutto, a 6.5+ oz portion of fish with almost a whole stalk of baby bok choy and a light import lager came in at under 500 calories.