Wash and trim the tops and tails from all the green beans and cut into 2" pieces. Set aside.
Using a wide skillet or a wok, bring 2 tablespoons of vegetable oil to high heat. Sauté the marinated chicken until light golden brown on both sides of the slices, about 3 to 5 minutes tops. Transfer the browned chicken to a plate or bowl for later.
1½ tablespoon vegetable oil, 1½ lbs chicken breasts
Add the second 2 tablespoons of vegetable oil to the skillet or wok and bring to high heat. Add the cut string beans and minced garlic and stir fry for 2 minutes, then cover for 2 minutes on lowered heat.
1½ tablespoon vegetable oil, 1½ lbs string beans, 3 cloves garlic
Uncover and add the slivered onion, chicken broth, soy sauce, cornstarch, and mirin. Cook for 1 minute stirring to mix and let the sauce thicken.
½ medium onion, ¾ cup chicken broth, 2 tablespoon soy sauce, 2 teaspoon cornstarch, 2 tablespoon Mirin
Return the browned chicken slices back to the skillet or wok, and add the remaining toasted sesame oil. Stir fry on medium for 1 minute. Remove from heat and serve immediately.
1 teaspoon toasted sesame oil