Coat the Chicken: Combine the chicken cubes and fish sauce in a medium bowl. Add the cornstarch and toss thoroughly until the chicken is evenly coated. Set aside.
Heat the Skillet: Heat 1/2 tablespoon of the oil in a large nonstick skillet over medium-high heat.
Sauté the Aromatics: Add the minced garlic and fresh ginger to the hot skillet. Stir-fry for 30 seconds.
Cook the Vegetables: Add the bell peppers, chopped red chili pepper if using, and pineapple chunks. Stir-fry for 3–5 minutes, or until the peppers are tender-crisp.
Add the Sauce: Pour in the Thai sweet chili sauce. Cook and stir until the sauce is heated through, then remove the pepper and pineapple mixture from the skillet. Cook the Chicken: Add the remaining oil to the skillet. Add the coated chicken and stir-fry for 5 minutes, or until the chicken is cooked through. Combine Everything: Return the cooked bell pepper and pineapple mixture to the skillet. Stir-fry until the chicken, vegetables, pineapple, and sauce are well blended. Garnish and Serve: Finish the stir fry with cilantro leaves and serve.