In a bowl, combine the flour, salt, and black pepper. Dredge the pork pieces in the flour mixture until well coated.
Heat the vegetable oil in a large skillet or wok over medium-high heat. Add the coated pork pieces in batches and fry until golden brown and cooked through, about 4-5 minutes per batch. Remove and drain on paper towels.
In the same skillet, add the onion and bell pepper, and sauté for about 2-3 minutes until they start to soften.
In a separate bowl, mix together the sugar, apple cider vinegar, ketchup, and soy sauce. Pour this mixture into the skillet with the vegetables and bring to a simmer.
Add the pineapple chunks to the skillet and stir well.
Slowly add the cornstarch mixture to the sauce, stirring constantly until the sauce thickens, about 1-2 minutes.
Return the fried pork to the skillet and toss to coat in the sweet and sour sauce. Cook for an additional 2-3 minutes to heat through.
Serve hot over steamed rice or noodles.