Thai Chicken with Basil

Name: Thai Chicken with Basil
Ingredients: 1 1/3 pounds boneless, skinless chicken breasts (about 4), cut into 1-by-2-inch pieces; 2 tablespoons nam pla or nuoc mam ( Asian fish sauce ); 1 1/2 tablespoons soy sauce; 1 tablespoon water; 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 garlic cloves, minced; 1 1/2 cups lightly packed basil leaves, divided