In medium-sized jug, combine stock, tamari, and vinegar, then add Thai chili, lemongrass, and gingerroot pieces. Set aside.

In 10-inch skillet, heat sesame oil on medium-high. Sear bok choy for 2 minutes, until golden brown on all sides, turning as necessary. Reduce heat to medium and pour in stock mixture. Simmer for 2 minutes. Reduce heat to medium-low so that liquid is at a gentle simmer. Add fish pieces and cook for 5 to 7 minutes, or until sections of sablefish begin to flake apart but pieces remain intact.

With tongs, remove bok choy and place on warmed plates. With slotted spoon or fish spatula, remove sablefish and place atop bok choy on each plate. Spoon a ladleful of cooking stock overtop, sprinkle with green onions, and serve.