In a small bowl, whisk together the oyster sauce, light soy sauce, dark soy sauce, fish sauce, and sugar. Set aside.
Heat oil in a wok till it starts smoking. Add the half of the garlic, ginger, onion and fresh chilli. Stir-fry rapidly for about 20-30 seconds until very fragrant. Be careful not to burn it!
Add the minced chicken to the wok. Break it up with your spatula and stir-fry vigorously. Cook until the chicken turns pale white, releases juices, and is mostly cooked through. This should take 5 to 6 minutes.
Pour the prepared sauce mixture, green onion, and remianing garlic over the cooked chicken.
Continue to stir-fry on high heat, tossing everything together. The sauce will thicken slightly and coat the chicken. Cook for another 1-2 minutes until the sauce is well incorporated and slightly reduced. If you want it extra saucy, at this stage, add the stock plus cornstarch slurry.
Add the generous handful of basil leaves. Toss them quickly through the hot chicken until they just wilt. The residual heat will cook them perfectly without overcooking.
Serve Thai Basil Chicken with jasmine rice.