Steaming fish “en papillote,” or in a sealed parchment paper bag, is a fantastic cooking method because it’s forgiving (less chance of severely overcooking), it’s great for reducing fishy aromas while cooking, and it’s... a perfectly portioned meal all in one neat package.
Whisk all sauce ingredients together. Taste and adjust seasonings. Set aside.
Season fish with a pinch of salt and pepper on both sides. On two pieces of parchment paper, divide up bok choy, mushrooms, fish, scallions, and pour 1/4 of the sauce over the top of each, reserving the other half for later. Top with orange slices. Create the parchment paper "bag" by folding the left and right piece of the paper toward the center, creasing the edges shut as tightly as you can. Then, fold and crease the top and bottom edges toward the center. (The first fold will want to fall apart until you get the top and bottom folded.)
Place packages on a sheet pan and bake fish for 10 minutes. Remove from the oven and let sit for up to a minute. Open package carefully (there will be steam) and check that internal temperature is at 145F. If your fish isn't completely cooked, fold the paper shut again and let steam for a few more minutes, only putting back in the oven if it's extremely underdone.
When your fish is fully cooked, transfer food from package to a plate or enjoy from the package. Pour the remaining sauce over the top and serve.
Alternatively, you can bake everything in a glass baking dish and cover tightly with foil. Don’t wrap the fish only in foil because the citrus in this recipe will react with the foil and affect the taste.