Bok choy, also known as Chinese cabbage, has long, white stalks and deep green, crinkled leaves. Bok choy has a sweet and mild cabbage like flavor. Choose bok choy with fresh crisp leaves and firm stalks.
Rinse mahi mahi and pat dry with paper towels. Cut fish into 1 1/4 inch pieces. Combine fish, sake, 1/4 teaspoon salt, and pepper in a large bowl. Set aside.Peel ginger and slice thinly; cut slices into very fine julienne strips. Separate bok choy into individual leaves. Rinse well and pat dry with paper towels. Cut bok choy stems diagonally into 1/2 inch slices. Cut leaves crosswise into 1 inch slices. Set bok choy stems in separate pile from leaves.Cut onions into 1inch pieces. Finely chop garlic. Combine teriyaki, water, and cornstarch in a cup; stir until smooth. Set aside.Heat the wok over high heat about 1 minute or until hot. Drizzle 1 tablespoon oil into the wok and heat 30 seconds. Add bok choy stems and stir fry about 2 minutes or until crisp tender. Add bok choy leaves and garlic; stir fry until tender. Sprinkle with the remaining 1/4 teaspoon of salt. Transfer to a serving plate and keep warm.Drizzle the remaining 3 tablespoons of oil into the wok and heat for 30 seconds. Add fish and stir fry gently about 4 minutes or until fish is lightly browned and flakes easily when tested with a fork. Place the fish over the bok choy.Add onions and ginger to the wok. Stir cornstarch mixture until smooth and add to the wok. Stir fry until sauce boils and thickens. Spoon sauce over fish.Serve with rice. Garnish if desired.