Thinly slice half of the ginger; finely grate enough of the remaining ginger to make 2 tsp (10 mL). Cut white parts of onions into 1-inch (2.5 cm) lengths; finely slice green parts and place in small bowl of ice water. Set aside.

In saucepan just large enough to hold salmon in one layer, pour in enough water to come 2 inches (5 cm) up side of pan; add sliced ginger, white parts of onions, 4 tsp (20 mL) of the vinegar and 1 tsp (5 mL) salt. Bring to boil; reduce heat and simmer for 5 minutes. Add salmon; cover and cook at bare simmer just until fish flakes easily when tested, 4 to 6 minutes. Remove fish with slotted spoon; let cool. (Make-ahead: Cover and refrigerate for up to 1 day.)

Drain green parts of onions. Mix mayonnaise with grated ginger, onions, remaining vinegar, sesame oil and ¼ tsp (1 mL) pepper. Serve with salmon.