Thoroughly clean and rinse the raw chicken. Set it aside.
Combine vegetarian oyster sauce, tamari, vegan fish sauce, and brown sugar in a mixing bowl. Whisk to combine. Set aside.
Heat oil in a skillet. Add the chicken.
Cook for 3-4 minutes until it starts to brown and no longer pink. Use a spoon to move around the chicken so that it’s evenly cooked.
Once done, remove the chicken from heat and set aside.
Heat sesame oil in the same skillet. Add green beans to the skillet and cook for 4-5 minutes, stirring them occasionally.
Once the green beans are fork-tender, add bell peppers, and garlic to the skillet. Mix to combine.
Stir in onions.
Cook for additional 3-4 minutes until the onions begin to soften.
Return the chicken to the skillet. Move the chicken and veggies to the side of the skillet.
Stir in the sauce mixture. Mix everything together.
Drain the excess liquid. Cook for another 2 minutes until the sauce starts to thicken.
Remove the skillet from heat. Add chopped green onions and Thai Basil leaves.
Give it a toss. Transfer Thai basil chicken to a bowl and serve hot.