Step 1: Marinate the Chicken
1. Prepare Marinade: In a large bowl, combine gochujang, gochugaru, soy sauce, sesame oil, sugar, minced garlic, minced ginger, black pepper, and rice vinegar. Mix well to form a thick marinade.
2. Marinate Chicken: Add the chicken pieces to the marinade, ensuring they are evenly coated. Cover and refrigerate for at least 30 minutes (or up to 2 hours for more flavor).

Step 2: Cook the Chicken
1. Heat Oil: In a large skillet or wok, heat the vegetable oil over medium-high heat.
2. Sauté Onions and Peppers: Add the sliced onion and bell pepper to the skillet. Sauté for about 3–4 minutes until they begin to soften.
3. Add Chicken: Add the marinated chicken to the skillet, spreading it out in a single layer. Cook for about 5–7 minutes, stirring occasionally, until the chicken is cooked through and slightly caramelized.

Step 3: Garnish and Serve
1. Garnish: Once the chicken is cooked, remove it from heat. Garnish with chopped green onions and sesame seeds.
2. Serve: Serve hot with steamed rice, lettuce wraps, or over noodles.