In a medium bowl, season the chicken pieces with salt, pepper, and cornstarch (if using). Set aside.
Heat 2 tablespoons vegetable oil in a wok or large skillet over medium-high heat.
Add the chicken and stir-fry for 4–5 minutes until golden brown and cooked through. Remove and keep warm.
Add carrots, green and red bell peppers, and garlic to the wok; cook and stir for 1–2 minutes until veggies are crisp-tender.
Stir in pineapple chunks, then pour in brown sugar, rice vinegar, and soy sauce. Mix well.
Return the chicken to the pan and toss everything together until coated in the glossy sauce, about 2 minutes.
Serve hot over steamed rice or noodles and garnish with green onions or sesame seeds if desired.