In a large bowl, whisk together the egg, 2 Tbsp cornstarch, and a pinch of salt and pepper. Trim any excess fat from the chicken thighs, then cut them into small 1 inch pieces. Toss the chicken in the egg and cornstarch mixture.
Heat a large skillet with 2 Tbsp of vegetable oil over medium-high heat. When the oil is very hot (it should look wavy on the surface), add the chicken and all of the egg mixture. Cook the chicken, stirring only occasionally, until it is golden brown and cooked through (about 7-10 minutes). Drain off the excess oil, if desired.
While the chicken is cooking, prepare the sauce. In a small bowl stir together the soy sauce, water, sesame oil, brown sugar, rice vinegar, fresh ginger, minced garlic, cornstarch, and sesame seeds. (You can grate the ginger with a small-holed cheese grater).
Once the chicken is cooked through and golden brown, pour the sauce over top. Toss the chicken to coat in the sauce. The sauce will begin to thicken as soon as it hits the hot skillet. Once the chicken is coated and the sauce thickened, turn off the heat.
Serve the chicken over a bed of rice and sprinkle the sliced green onions over top.