How to Make It
Toss fish with 1/2 cup cornstarch in a bowl, shaking off excess. Place fish on a baking sheet; chill 10 minutes.
Whisk together stock, soy sauce, vinegar, and remaining 2 teaspoons cornstarch. Set aside.
Heat 2 tablespoons oil in a large nonstick skillet over high. Add half of fish to pan. Cook without stirring, until browned and crispy, about 4 minutes. Turn fish, and cook until fish flakes with a fork, 1 to 2 minutes. Remove fish from skillet. Repeat process with 2 tablespoons oil and remaining fish. Wipe skillet clean.
Add remaining 1 tablespoon oil to skillet. Add garlic and ginger; cook over high, stirring constantly, until golden, 5 to 10 seconds. Add scallions and bell pepper; cook 1 minute. Remove skillet from heat. Add soy sauce mixture, crushed red pepper, and fish; stir gently to coat (sauce will thicken). Serve over rice, if desired.