1 In a food processor, mince the garlic. Add the ginger and coarsely chop. Add the scallions and coarsely chop. In a small bowl, combine 3 tablespoons of the cornstarch with the pineapple juice.
2 In a small skillet or saucepan, warm 2 teaspoons of the oil over medium-high heat until hot but not smoking. Add the garlic, ginger and scallions, and cook, stirring, until the garlic begins to brown, about 3 minutes.
3 Add the cornstarch-pineapple juice mixture, the vinegar, 2 teaspoons of the soy sauce, the ketchup, sugar and pepper, and bring to a boil over medium-high heat. Reduce the heat to low, cover and simmer while you prepare the fish.
4 In a shallow bowl, beat the egg white with the remaining 1 tablespoon cornstarch and 2 teaspoons soy sauce.
5 Cut the fish into 1/2 inch-wide strips. Add the fish to the egg white mixture and toss to coat well. Place the fish strips on a plate and sprinkle them with the sesame seeds, turning the fish to coat all sides.
6 In a medium nonstick skillet, warm the remaining 1/2 cup oil over medium-high heat until hot but not smoking. Add the fish in batches and cook until golden, about 3 minutes per side. As they are done, place the fish on a paper towel-lined plate in a low oven to keep warm.