Step 1: Prepare the Chicken
In a medium bowl, combine the cubed chicken with the fish sauce and toss until evenly coated.
Sprinkle the cornstarch over the chicken and toss again until every piece is lightly coated. Let the chicken rest while preparing the vegetables.
Step 2: Cook the Vegetables
Heat ½ tablespoon of oil in a large non-stick skillet or wok over medium-high heat.
Add the minced garlic and fresh ginger, stirring constantly for about 30 seconds until fragrant.
Add the diced bell peppers, chopped chili pepper if using, and fresh pineapple chunks.
Stir-fry for 3 to 5 minutes until the vegetables are tender-crisp while still retaining their vibrant color.
Pour in the Thai sweet chili sauce and stir until everything is evenly coated and heated through.
Transfer the vegetable mixture to a plate.
Step 3: Cook the Chicken
Add the remaining oil to the same skillet.
Place the chicken in a single layer and stir-fry for about 5 minutes until fully cooked and lightly golden on the outside. The internal temperature should reach 165°F (74°C).
Step 4: Finish the Stir Fry
Return the cooked vegetables and pineapple to the skillet.
Toss everything together until evenly coated in the sauce and heated through.
Garnish the Pineapple Chicken Stir Fry with fresh cilantro leaves and serve immediately.