In a bowl mix all the ingredients for the sauce. Set aside.
¼ cup tomato ketchup, ¼ cup brown sugar, ½ teaspoon salt, ¼ cup vinegar, 1 teaspoon cornstarch, ¼ cup water
Cut the vegetables and set them aside.
2-3 small bell peppers in assorted colors, 1 medium red onion, 1 cup pineapple chunks
Place the fish slices in a bowl or plate and season with salt, pepper, and lemon or calamnsi juice.
10.5 ounces fish fillet like cream dory, ½ teaspoon salt, ¼ teaspoon white ground pepper, 2 tablespoons lemon or calamansi juice
Heat enough oil for frying, about 2-3 inches, in a wok or deep pan over medium heat.
1-2 cups canola or corn oil for deep frying
While oil is heating up, sift together the flour, cornstarch, and baking powder in a bowl. Add the salt and mix. Add the egg and cold water and whisk into a smooth batter. (Remember to keep the batter cold.)
1 medium egg, ½ cup flour, ½ cup cornstarch, ½ teaspoon salt - adjust to taste, ½ teaspoon baking powder, ¼ cup ice cold water
Check if oil is hot enough by dipping a chopstick, it should bubble immediately.
Take a piece of fish fillet and dredge it in cornstarch covering both sides then dip it in the batter and then deep fry in the hot oil. Deep fry the meat in batches if needed, do not overcrowd. Turn them several times using a slotted spoon to achieve even cooking. Cook until golden brown.
¼ cup cornstarch for dredging
Remove cooked fish fillet using a spider or slotted spoon and place it on a plate for now.
Empty the oil from the wok, no need to wipe it clean. Turn on the heat to medium-high and add 1 tablespoon of oil. Add onions and bell peppers and cook for some seconds. Add the pineapple chunks and cook for some seconds more.
2-3 small bell peppers in assorted colors, 1 medium red onion, 1 cup pineapple chunks
Pour the sauce into the wok and simmer until it has thickened to a paste-like consistency for about a minute or two.
Add back the fried fish fillet. Gently toss and mix for some seconds just until everything is coated with sauce. Transfer to a serving dish.