Score the fish by making an slashes about half an inch deep along its length on both sides. Season all over with salt and pepper, including the cavity.
In a small sauce pan, mix together the pineapple juice, Dijon mustard, lemon juice and one tablespoon starch. Add sugar, one teaspoonful at a time, until you reach a balance of sweetness and sourness that you prefer. Cook over medium heat, stirring occasionally, until thick and clear. To make sure that no floury taste remains, lower the heat, cover and simmer for about 5 minutes. Set aside.
Heat enough cooking oil in a frying pan to reach a depth of three inches.
Sprinkle the remaining starch on both sides of the fish. Over high heat, fry the fish until a crisp coating has formed, flipping halfway through the cooking to fry both sides evenly.
Remove the fish from the oil and drain on paper towels or place on a wire strainer to allow excess oil to drip off.
Transfer 1 tablespoonful of the oil to a small frying pan. Pan fry the onion, garlic, carrot, bell pepper and ginger for about 45 seconds or just until they start to soften.
To assemble: Place the fish on a serving platter. Arrange the vegetables on top. Pour the sauce over the fish and vegetables. Serve at once.