Preheat oven to 425°F (220°C). Whisk together vinegar, sugar, soy sauce and ginger in a shallow dish. Place fish into marinade, turning to coat. Set aside.
Fold each sheet of foil or parchment in half then open flat so each piece looks like an open book. Place cooked rice on one side of each book, avoiding the fold. Top rice with fish. Divide green onions and carrots among the packets, and drizzle with the vinaigrette and remaining marinade. Fold the empty side over the ingredients. Tightly seal the edges of foil or parchment.
Place packets on a baking sheet and bake in centre of oven until the fish is opaque and the vegetables are tender-crisp, about 8 min. (for foil) or 10 min. (for parchment paper). Carefully open one packet (watching out for steam) to check for doneness; fish should flake with a fork. Let stand 5 min. before serving.
Parchment paper is traditionally used to make this dish and we love its presentation, but foil offers a slightly faster cooking time, which is ideal for weeknights.