Heat oil in wok over high heat. Add onion and cook for 3-4 minutes.
Add garlic and cook another 1 minute.
Toss chicken in cornstarch and add to the wok. Cook for about 5 minutes, stirring frequently until almost cooked through.
Add Serrano pepper, green Curry paste, fish sauce, Sherry, brown sugar and soy sauce. Stir and cook about 2 minutes until reduced slightly.
Add chicken stock and cook for another 3 minutes until sauce thickens a bit.
Stir in Holy Basil and serve immediately over your Jasmine Rice.