In a small bowl, mix wine vinegar, sesame oil, ginger, garlic, onion, brown sugar, soy sauce, and water.
Heat your skillet to medium heat and pour in the sauce. Bring sauce to a light simmer and let simmer for about 10 minutes until sauce darkens. Then, carefully pour sauce back into the bowl and set aside for later.
In a large mixing bowl, add chicken and cornstarch and toss until chicken is fully coated. You may need to add an additional tablespoon or two of cornstarch.
In a small bowl, whisk egg. Pour egg over the chicken and gently mix chicken to coat.
In the same skillet you cooked the sauce in, add oil and heat to medium-high heat. Once hot, carefully add chicken to the skillet. Gently spread chicken out into an even layer and let cook until bottom is golden. Don't panic, the chicken will look weird at this point but it will all come together.
Once the bottom of the chicken is golden, flip chicken over, gently breaking pieces apart. Cook for another 5-7 minutes on the other side until golden. Then, turn heat to low and let cook for another 10 minutes, stirring occasionally.
Drain excess oil from pan (if any) and turn heat up to high. Pour sauce over the chicken and begin stirring frequently for a few minutes.
Serve with a sprinkle with fresh chopped green onions and sesame seeds (optional). Enjoy!