Heat oven to 400° F. Tear off four 15-inch squares of parchment paper or aluminum foil and arrange on 2 baking sheets.
Divide the bok choy and bell pepper evenly among the squares, place the halibut fillets on top, sprinkle with black pepper, and top with the scallions and zest.
In a small bowl, combine the soy sauce, vinegar, oil, and ginger. Spoon the mixture evenly over the halibut.
Top each fillet with another square of parchment or foil and fold the edges over several times to seal. Bake for 15 minutes.
Transfer each packet to a plate. Serve with a knife to slit open the package. Be careful of the steam that will escape.