Preheat oven to 400°F.
Cut four 9 x 11-inch pieces of parchment paper. Place one-quarter of the scallions, 1 bok choy, 1/4 of the red pepper slices, and 1/8 of the sliced ginger in the lower third section of each piece.
Place 1 cod fillet on each bed of veggies and season the fish with salt and pepper. Arrange the remaining sliced ginger on top of the fish and pour 1 teaspoon each of the sesame oil, mirin, and tamari over each fillet.
Fold the top half of the parchment paper over the fish to make a rectangle; then, starting with one side, tightly roll up the parchment paper. Repeat with the other two edges, then place the parcels on a baking sheet and bake for 15 to 20 minutes, until flaky and cooked through.
Carefully tear open the packages to serve over brown rice or quinoa.