Add cubed chicken thighs to a large bowl. Toss to fully coat the chicken in cornstarch. In a small bowl, whisk the egg. Pour egg mixture over the chicken and gently toss to fully coat.
In a small saucepan over medium heat, add all sauce ingredients and whisk to fully mix. Add sliced onion and bring to a simmer until sauce reduces to about half.
In the meantime, heat a large skillet to medium-high heat and add the oil. Gently add chicken in an even layer to the hot pan. Let cook undisturbed until bottoms are nice and golden. Then, flip chicken over, breaking apart any pieces that are sticking together. Continue cooking until bottoms are golden.
Turn up the heat on the chicken skillet to high heat and slowly begin adding sauce while stir frying until all sauce is incorporated.
Serve chicken immediately over steamed rice or with steamed vegetables.