Heat oven to 400ºF (200ºC).
Cut 4 pieces of parchment paper so they are roughly 12”x14”. Fold each piece of parchment paper in half and cut a semicircle into the paper so it makes a full circle when opened.
Working one at a time, arrange the bok choy on one side of the parchment paper and drizzle with soy sauce. Top with one slice of fish and season with salt.
Working from one end, begin to tightly fold the edges of the parchment paper so that the package is completely sealed. Repeat with the remaining parchment paper, bok choy and fish.
To bake the fish, add the packages to a rimmed baking sheet and bake until the fish is tender, about 8 to 10 minutes.
Open the packages and serve garnished with green onion and sesame seeds with a side of rice.