Cut chicken into 1-inch square pieces. Place in a medium bowl. Stir in lemon grass, 2 tbsp. Of the soy sauce, and salt. Mix well. Cover and refrigerate 2 hours or longer. Bring to room temperature before cooking. Combine chicken broth, remaining 2 tbsp. Soy sauce, sugar, and chili in another small bowl; set aside. Cut onion in half lengthwise; cut each piece into 4 wedges; separate layers. Heat wok over medium-high heat. Add oil and heat. Add minced onion, gingerroot, and garlic; stir-fry 15 seconds. Do not burn. Increase heat. Add chicken; stir-fry 2 minutes. Add sliced onion; stir-fry 1 minute. Add chicken broth mixture; cook, stirring, 1 minute. Stir cornstarch mixture to blend in cornstarch that has settled. Add to the chicken mixture; cook until sauce is thickened, about 30 seconds. Spoon onto a heated platter; serve at once.