In a large bowl, combine soy sauce, brown sugar, apple cider vinegar, minced garlic, minced ginger, and red pepper flakes (if using). Add the chicken pieces and marinate for at least 15 minutes.

In a large skillet or wok, heat half of the vegetable oil over medium-high heat.

Remove the chicken from the marinade (reserve the marinade for later) and add it to the skillet. Cook until browned on all sides and cooked through, about 6-8 minutes. Remove the chicken from the skillet and set aside.

In the same skillet, add the remaining oil and pour in the reserved marinade. Bring it to a boil, then reduce heat to simmer for about 5-7 minutes until slightly thickened.

Return the chicken to the skillet and toss to coat in the thickened sauce. Cook for an additional 2-3 minutes.

Garnish with chopped green onions and serve over rice, pasta, or on its own!