1. Combine first 4 ingredients, stirring until sugar dissolves; set aside.
2. Combine vinegar and wasabi in a small bowl, stirring until smooth. Spread wasabi mixture evenly over top of salmon.
3. Line a bamboo steamer with cabbage leaves. Place salmon, wasabi side up, over cabbage in steamer basket; cover with steamer lid.
4. Add water to a large skillet to a depth of 1 inch; bring to a boil. Place steamer in pan; steam salmon 8 minutes or until fish flakes easily when test with a fork or until desired degree of doneness. Remove salmon from steamer; discard cabbage. Serve salmon with sauce; sprinkle with onions, if desired.