Heat oil in a large skillet over medium heat. Add chicken and season with salt and pepper. Cook until golden and cooked through, about 6–7 minutes.
Add garlic and bell peppers to the skillet. Sauté for 2–3 minutes until fragrant and slightly softened.
Pour in sweet chili sauce, soy sauce, and rice vinegar. Stir well to coat the chicken and peppers.
Stir in the cornstarch slurry and cook another 2 minutes until the sauce thickens and becomes glossy.
Remove from heat, garnish with sesame seeds and green onions, and serve hot over rice or noodles.