Chop the onion fine. Mince the garlic. Slice the raw, thawed chicken breast into thin strips.

Whisk onion, soy sauce, brown sugar, garlic, sesame oil, sesame seeds, cayenne pepper, salt, and black pepper together in a bowl until sauce is smooth.

Cook and stir chicken and sauce together in a large skillet over medium-high heat until chicken is cooked through, about 15 minutes.