Place the soy, jelly and vinegar in a small saucepan over low heat, stirring until the jelly melts, about 1 minute.
Let the glaze cool, and then place in a resealable plastic bag with the cod. Marinate in the refrigerator about 1 hour.
Preheat the broiler. Line a baking sheet with foil.
Remove the cod from the marinade and place the marinade in a small saucepan.
Transfer the cod to the prepared baking sheet. Bring the glaze to a boil and turn off the heat.
Remove 2 tablespoons of the glaze from the pot, reserving the rest for serving, and brush the cod with the glaze. Broil the cod about 5 inches from the heat, brushing with additional glaze halfway through, until just cooked through, 5 to 7 minutes depending on the thickness of the fish. It should feel just firm to the touch.
Stir together the scallions and zest and sprinkle over the fish.