In a medium bowl, combine the chicken, mirin, salt, and cornstarch. Toss until evenly coated and let sit for 15 minutes while you prepare the remaining ingredients.
In a measuring cup or bowl, whisk together the sauce ingredients. Set aside.
Heat 1 tablespoon oil in a large skillet or wok over medium-high heat. Add the chicken in a single layer and cook for 3-4 minutes, stirring occasionally, until mostly cooked through. Transfer to a plate.
Add the remaining tablespoon of oil to the skillet. Add the onion, celery, and carrots. Stir fry for 4-5 minutes until the vegetables begin to soften but still have a little bite.
Add the garlic and cook for 30 seconds, stirring constantly.
Give the sauce a quick whisk and pour it into the skillet. Bring to a simmer.
Return the chicken to the skillet. Cook for 2-3 minutes, stirring frequently, until the sauce thickens and the chicken is fully cooked.
Serve immediately over hot white rice.