Sweet and sour fish in 20 mins (糖醋鱼)

Name: Sweet and sour fish in 20 mins (糖醋鱼)
Prep time: 5 min
Cook time: 15 min
Total time: 20 min
Cuisine: Chinese
Ingredients: 1 whole rainbow trout, , gutted and descaled (about 600g, see note 1 for other options); 1 tablespoon cornstarch (or potato starch); 2 tablespoon cooking oil; 2 stalk scallions (chopped); 2 slices ginger; 4 cloves garlic, sliced; 1 tablespoon Shaoxing rice wine (or white wine); 1 tablespoon light soy sauce; 3 tablespoon white rice vinegar (or cidre vinegar); 1½ tablespoon sugar; 3 tablespoon tomato ketchup; 120 ml water (½ cup); ¼ teaspoon white pepper; ½ teaspoon salt; Coriander, chopped