How to Make It
Preheat oven to 275°F. Whisk together first 6 ingredients (through salt) in a small bowl.
Place fish, skin side down, on a foil-lined baking sheet. Brush fish with 3 tablespoons oyster sauce mixture; bake at 275°F until a thermometer inserted in thickest portion reaches 120°F, about 12 minutes. Transfer to a serving plate.
Heat oil in a large skillet over medium-high. Add bok choy and garlic; cook, stirring often, until slightly softened, about 2 minutes. Add 2 tablespoons water; cover and steam until tender, about 3 minutes. Remove from heat. Add remaining 2 tablespoons oyster sauce mixture; toss to coat. Serve with fish; garnish with scallions.