If using ti leaves, cut each leaf in half along center rib and discard rib. If using banana leaves, soak in warm water to soften. Tear 12 long, narrow strips from 1 or 2 leaves and boil 30 seconds; drain. Cut remaining leaves with the grain into 24 strips, each about 12 in. by 3 in.
Cut fish into 12 equal pieces (about 2 in. by 2 in. each) and sprinkle both sides with coarse kosher salt. Chill 30 minutes.
Meanwhile, cut carrot, bell pepper, and green onions into 2-in. lengths and then cut into thinnest possible slivers. Melt butter in a frying pan over medium heat. Add vegetables and ginger and sauté, stirring often, until softened but not browned, 3 to 4 minutes. Remove from heat.
Arrange 2 strips of ti or banana leaves, shiny sides down, on top of one another in a cross shape. Place 1 mahimahi piece in center of cross, then top with a generous tbsp. vegetable mixture. Beginning with lower strip, fold leaves over filling, alternating strips and using each new strip to fold loose end of previous strip over filling. Tuck last strip beneath packet, then tie closed with boiled leaf strips. Repeat with remaining leaf strips, fish, and vegetables.
Set a rack or steamer basket in a large pot and add water to about 1/2 in. below top of rack. Bring water to a boil over high heat. Place fish packets in a single layer on rack or in basket (steam in batches if necessary). Cover pot tightly and steam until fish is just barely opaque in the center, 6 to 10 minutes (do not overcook; cut to test). Tip packets to drain any water. Serve hot or warm, with lime wedges and Hawaiian red clay salt.
Note: Nutritional analysis is per serving.