Special equipment: bamboo steamer basket
Bring a wok or large pot filled halfway with water to a boil and reduce to a simmer. Divide the fish between 2 plates that will fit into the basket layers of a bamboo steamer. Sprinkle the fish with sea salt and freshly ground white pepper. Drizzle 1 to 2 tablespoons of the pineapple juice over each plate of fish. Place the plates into the steamer, cover and set on top of the simmering water. Steam the fish until the flesh turns white and pulls apart easily, 7 to 8 minutes. Remove the steamer basket from the wok and set aside, keeping the fish covered.
For the sauce: Place a wok over high heat and when the wok is hot, add the oil. Add the garlic and ginger and cook until fragrant, less than 1 minute. Add the red peppers and onions and toss to cook until tender, another minute. Add the pineapple chunks and their juice. Add the vinegar, soy sauce and brown sugar. Cook until the sauce thickens a bit 1 to 2 more minutes.
Remove the fish to a platter. Season the sauce if needed and pour over the fish. Garnish with the sliced scallion.