Peel back tough green husks from 1 stalk lemongrass. 

Finely grate enough of the tender core to equal 1/2 teaspoon; place in a small bowl. 

Whisk in vinegar, soy sauce and water. Set aside.

Chop all remaining lemongrass (including husks) into 2-inch pieces and place in the bottom of a steaming pot. 

Add enough water to come just below the bottom of the steaming rack; place over high heat and bring to a boil. 

Lower heat and simmer, covered, 5 minutes. 

Uncover pot, place barramundi on rack and sprinkle with mushrooms, green onions, ginger and shallot. 

Cover and steam just until fish is opaque and flakes when tested with a fork, 6 to 7 minutes. 

Transfer fish and vegetables to plates.

 Drizzle with vinegar mixture and sprinkle with sesame seeds.