1 1/3 pounds boneless, skinless chicken breasts (about 4), cut into 1-by-2-inch pieces
2 tablespoons Asian fish sauce (nam pla or nuoc mam); see Note
1 1/2 tablespoons soy sauce
1 tablespoon water (more water and maybe less soy/fish sauce)
1 1/2 teaspoons sugar
2 tablespoons cooking oil
1 large onion, cut into thin slices
3 fresh red chiles, seeds and ribs removed, cut into thin slices, or 1/4 teaspoon dried red-pepper flakes
3 cloves garlic, minced
1 1/2 cups lightly packed basil leaves