Season the cubed pork with salt and pepper. Toss in cornstarch until each piece is evenly coated.
Heat oil in a wok or large skillet over medium-high heat. Fry the pork in batches until golden and crispy, about 3–4 minutes per side. Drain on paper towels.
In the same pan, add a touch more oil if needed. Stir-fry garlic, onion, and bell peppers for 2 minutes until they start to soften.
Whisk together apple cider vinegar, sugar, ketchup, soy sauce, and pineapple juice in a small bowl.
Return pork and pineapple chunks to the pan. Pour in the sauce, tossing to coat. Simmer for 3–4 minutes until the sauce thickens and glazes the pork.
Spoon over steamed rice or toss with noodles. Garnish with sliced green onions or sesame seeds for extra flair.