**Prepare the Chicken:** In a bowl, mix flour, cornstarch, salt, and pepper. Dip the chicken pieces into milk, then coat in the flour mixture. Shake off excess.
**Fry the Chicken:** Heat vegetable oil in a large skillet over medium-high heat. Fry the chicken in batches until golden brown and cooked through, about 4–5 minutes per batch. Drain on paper towels.
**Make the Sauce:** In a small bowl, whisk together sweet chili sauce, pineapple juice, honey, soy sauce, and rice vinegar. Heat the sauce in a skillet over medium heat until it begins to simmer. Add the cornstarch slurry and stir until the sauce thickens.
**Stir-Fry the Vegetables:** In the same skillet, lightly sauté pineapple chunks, red bell pepper, and red onion until slightly softened but still crisp, about 2–3 minutes.
**Combine and Serve:** Add the fried chicken to the skillet with the sauce and vegetables, tossing to coat evenly. Garnish with chopped green onions and serve immediately.
