Marinate chicken in fish sauce, soy sauce, water, and sugar.
Heat the oil over medium-high heat. Add the onion and cook, stirring, for 2 minutes. Stir in the chiles and garlic; cook, stirring, 30 seconds longer
Remove the chicken from the marinade with a slotted spoon and add it to the hot pan. Cook until almost done, stirring, about 3 minutes. Add the marinade and cook 30 seconds longer. Remove from the heat and stir in 1 cup of the basil. Serve topped with the remaining basil