How to Make It
In a medium saucepan, bring the soy sauce, mirin, sugar and vinegar to a boil. Simmer over moderate heat until the glaze is reduced to about 1 1/2 cups, about 10 minutes.
Meanwhile, in another medium saucepan, combine the dashi with 1 1/2 cups of water and bring to a boil. Add the carrots and leeks and simmer just until crisp-tender, about 1 minute. Remove from the heat and let stand.
Preheat the broiler and position a rack 6 to 8 inches from the heat. Arrange the trout fillets on a sturdy baking sheet, skin side up, and broil for about 2 minutes, until the skin begins to sizzle. Carefully peel off the skin. Brush the fish with some of the glaze and broil for 3 minutes, brushing once or twice more, until the sauce is lightly caramelized. Turn the fillets, brush liberally with more glaze and broil for 3 minutes longer, brushing once or twice, until the trout is cooked through and lacquered.
Mound rice in shallow bowls and drizzle lightly with the glaze. Using a slotted spoon, add the carrots and leeks. Top with the fish and nori and spoon the dashi into the bowls. Serve with the remaining glaze on the side.