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 grill. 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. Grill 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 spring onions and zest and sprinkle over the fish. Serve with rice and the reserved glaze.