Prepare the Sauce: In a small bowl, whisk together pineapple juice, vinegar, ketchup, soy sauce, and brown sugar. Mix until smooth, then stir in cornstarch slurry. Set aside.
Cook the Shrimp: Heat 1 tablespoon of oil in a large skillet or wok over medium-high heat. Add shrimp and cook for 2–3 minutes on each side, until pink and opaque. Remove from pan and set aside.
Sauté the Vegetables: In the same skillet, add the remaining oil. Toss in garlic, onions (if using), and bell peppers. Stir-fry for 3–4 minutes until the peppers begin to soften but remain crisp.
Add Pineapple: Add the pineapple chunks and cook for another minute, stirring to combine.
Pour in the Sauce: Give the sauce a quick stir, then pour it into the skillet. Cook while stirring constantly until the sauce thickens and becomes glossy — about 1–2 minutes.
Combine Everything: Return the shrimp to the skillet. Toss everything together until the shrimp and vegetables are evenly coated in the sauce.
Finish and Serve: Remove from heat and serve hot. Garnish with green onions or sesame seeds if desired.