Start by slicing the chicken breast into thin strips for quick and even cooking. Mince the garlic, slice the bell pepper, trim and halve the green beans, and slice the red chili if using. Rinse the Thai basil leaves and pat dry.
In a small bowl, combine the soy sauce, oyster sauce, fish sauce, and sugar. Stir until the sugar dissolves.
Heat the vegetable oil in your wok or large skillet over medium-high heat. Add the minced garlic and sauté for about 30 seconds until fragrant but not browned. Toss in the chicken slices and stir-fry for 4-5 minutes until the chicken turns opaque and starts to brown slightly.
Add the sliced bell pepper and green beans to the pan. Stir-fry for another 3-4 minutes until the vegetables are tender-crisp, retaining their vibrant color and crunch.
Pour the sauce mixture over the chicken and vegetables. Stir well to coat everything evenly. Cook for another 2 minutes, allowing the sauce to thicken slightly and infuse the ingredients with flavor. Finally, toss in the fresh Thai basil leaves and sliced red chili if you want some heat. Stir until the basil wilts and releases its fragrant aroma.
Serve your Thai Basil Chicken Stir-Fry hot over a bed of fluffy jasmine rice. The rice will soak up the delicious sauce, making every bite a perfect harmony of textures and flavors.