Mix gochujang, sesame oil, soy sauce, honey, garlic, ginger, and black pepper in a bowl
Coat chicken pieces with the spicy sauce
Heat butter in a large skillet over medium-high heat
Cook chicken until golden and cooked through
Add sliced onions and cook until softened
Top with shredded mozzarella cheese
Cover and cook until cheese is melted
Garnish with green onions and serve