Season the fish with salt and pepper, then coat it with cornstarch or all-purpose flour. Fry over medium-high heat until crispy and golden brown. Transfer to a paper towel-lined plate or a wire rack.
Make the sauce by combining soy sauce, vinegar, sugar, cornstarch, pineapple juice, and ½ cup of water in a bowl. Set it aside.
In a separate pan, sauté ginger until aromatic. Then, add red pepper flakes, garlic, and onions. Sauté for another minute or two until softened.
Add the scallions, carrots, and bell peppers. Cook for a few minutes until the vegetables are slightly tender but still crisp. Season with salt and pepper.
Add the sauce and simmer for about 2 minutes until it slightly thickens. Add ground pepper as you like, or more water if needed. Taste and adjust as necessary.
Arrange the fried fish and vegetables on a plate, then pour the sauce over it. You can garnish with chopped cilantro or scallions. Serve and enjoy!