In a bowl, combine cornstarch, flour, salt, and pepper. Coat pork cubes thoroughly with the mixture.
Heat vegetable oil in a large pan or wok over medium-high heat. Fry pork cubes until golden brown and cooked through, about 5-7 minutes. Remove and set aside on paper towels.
Remove excess oil, leaving about 2 tablespoons in the pan. Add bell peppers, onion, and pineapple chunks. Stir-fry for 3-4 minutes until vegetables are tender-crisp.
Add minced garlic and ginger if using, and cook for 30 seconds until fragrant.
In a small bowl, whisk together brown sugar, vinegar, ketchup, and soy sauce. Pour into the pan with vegetables.
Return the fried pork to the pan and toss everything together until well coated. Cook for 2-3 minutes until heated through and sauce thickens slightly.
Serve immediately over steamed rice, garnished with sesame seeds or chopped green onions if desired.