In a large bowl, whisk together the flour, cornstarch, salt, and pepper. Gradually add water while whisking until the batter is smooth.
Heat oil in a frying pan or wok over medium-high heat. Dip chicken pieces into the batter and then carefully place them in the hot oil. Fry until golden brown and cooked through, about 4-5 minutes. Remove with a slotted spoon and set aside on a baking sheet.
In the same pan, sauté the onion and garlic until fragrant, about 1 minute. Add the bell pepper and pineapple chunks, and cook for another 2-3 minutes until slightly softened.
In a separate bowl, mix together the sugar, apple cider vinegar, soy sauce, and ketchup. Pour this mixture into the pan and bring to a simmer.
Add the fried chicken back into the pan and stir to coat with the sauce. Mix the cornstarch slurry and add it to the pan to thicken the sauce. Cook for an additional 2-3 minutes until the sauce is glossy.
Serve hot over steamed rice or noodles.