In a medium bowl, combine the chicken with the fish sauce. Add the cornstarch and toss until every piece is evenly coated. Let it rest for a few minutes.
Heat half of the oil in a large nonstick skillet or wok over medium-high heat.
Add the garlic and ginger. Stir-fry for about 30 seconds until fragrant.
Add the bell peppers, chopped chili pepper if using, and pineapple chunks. Cook for 3 to 5 minutes until the peppers are crisp-tender.
Pour in the sweet chili sauce and stir until everything is coated and heated through. Transfer the vegetable mixture to a plate.
Heat the remaining oil in the same skillet.
Add the chicken and stir-fry for about 5 minutes or until fully cooked and lightly golden.
Return the vegetable mixture to the skillet and toss everything together until evenly combined. This Pineapple Chicken Stir Fry is ready once everything is hot and well coated.
Garnish with fresh cilantro and serve immediately over steamed jasmine rice or noodles.