Make the marinade by combining the ingredients for the marinade in a large measuring bowl.
Whisk until well combined, then pour half of the marinade into a zip-lock bag to use as a marinade. Cover and reserve the remaining marinade for later use as a sauce.
Marinate the chicken by slicing the chicken into thin, bite-sized strips. Place the chicken into the zip-lock bag with the marinade, then seal and refrigerate for 2-3 hours.
Remove the chicken from the refrigerator 20 minutes before cooking.
Cook the stir fry by heating half of the oil in a large cast-iron skillet or wok. Add the onion and cook for 2 minutes.
Add the shiitake mushrooms and continue cooking for 1-2 minutes, stirring often.
Add the kale and minced garlic, then season with sea salt and freshly cracked pepper, to taste; toss to coat evenly, then cook, stirring constantly, for 1 minute.
Remove from the pan and pour into a bowl.
Add the remaining oil to the pan and heat over medium-high heat until hot. Add half of the chicken removed from the marinade to the hot skillet. Cook, stirring occasionally, for 2 minutes. Pour into the bowl with the veggies.
Add a bit more oil, if the skillet needs it, then add the remaining chicken removed from the marinade, discarding the zip-lock bag and marinade. Cook, stirring occasionally, for 2 minutes.
Pour into the bowl with the veggies.
Whisk the reserved marinade from earlier until well combined, then add to the skillet over medium heat. Whisk for a few seconds, then add the chicken, veggies, and any juices back into the skillet.
Stir to coat evenly and heat through.
Serve over garlic rice immediately and garnish with green onion. Enjoy.