Prep all ingredients in advance.
Heat oil over medium heat in cast iron skillet. (A wok would be even better, but we don't have one.)
Add garlic and chilies and stir fry until aromatic.
Increase heat. Add chicken and onions and stir-fry for about 3 minutes, separating the onions into rings as you stir. Add the sauce and cook for about 3 more minutes, until the chicken is tender and cooked through.
Turn off the heat, stir in the herbs and cook until just wilted over the residual heat.
Serve with rice.
This is very loosely adapted from Victor Sodsook's True Thai. The recipe calls for a whopping 8 serrano chilies, but Uly and I both remember that when the book came out 25 years ago, American serrano cilies were tiny! I don't think we ever used the full 8, but now we use far fewer than we used to.