In a small saucepan, combine the tamari and sugar. Cook over medium-high heat until reduced by half, stirring occasionally, about 3 minutes. Remove from the heat and cool slightly.
Prepare a grill.
Lay 1 taro leaf on a flat surface. Spoon about 1 tablespoon of the tamari reduction in the center and top with 1 mahi-mahi fillet. Top the fish with 1/4 of the onions, carrots, celery, green onions, ginger and garlic. Season the fish and vegetables with 1/4 teaspoon of the salt and completely wrap inside the leaf. Repeat with the remaining ingredients. Wrap each fish fillet inside another leaf and place, folded sides down on the hot grill. Grill for 7 minutes per side.
Remove from the grill and unwrap each bundle. Spoon 1/4 cup of the fruit reduction on each of 4 plates and top with the fish and vegetables. Serve immediately.
In a large saucepan, combine all the ingredients and cook over medium-high heat until reduced by half. Remove from the heat and cool for 5 minutes before serving.