In a large bowl, whisk together gochujang, soy sauce, mirin, sugar, garlic, ginger, and sesame oil.
Add chicken pieces to the marinade and toss until evenly coated; let marinate for at least 15 minutes.
Heat vegetable oil in a large skillet or wok over medium-high heat.
Add the marinated chicken and cook for 5 to 7 minutes until browned and mostly cooked through.
Add onion, cabbage, and carrot to the skillet.
Stir-fry for 4 to 5 minutes until vegetables are tender-crisp and chicken reaches an internal temperature of 165 degrees Fahrenheit.
Garnish with sliced green onions and serve.