In a medium bowl, whisk together soy sauce, brown sugar, and minced garlic. Add chicken pieces, coat well, cover, and refrigerate for at least 15 minutes.

Heat sesame oil in a large skillet over medium-high heat. Add marinated chicken in batches, cook for about 10 minutes until golden brown and fully cooked.

In the same skillet, pour in leftover marinade and chicken juices. Bring to a simmer and cook for about 3 minutes until thickened slightly.

Return chicken to the skillet, stir to coat with the sauce, and continue cooking on low heat for an additional 2-3 minutes.

Transfer Mongolian Chicken to a serving dish, garnish with chopped green onions, and serve hot over rice or noodles.