Whisk the gochujang, gochugaru, soy sauce, rice syrup, sesame oil, garlic, ginger, and pepper. Toss with the chicken and marinate for 20 minutes.
Cook:
Heat the oil in a wide skillet over medium heat. Add the chicken and sweet potato and cook for 6 minutes, turning the chicken once.
Add cabbage, onion, rice cakes, scallions, and water. Cover for 5 minutes, then uncover and stir-fry for 8 to 10 minutes until the chicken is cooked through and the vegetables are tender.
Let the sauce reduce just enough to coat the ingredients, then serve from the pan.
Notes
Use Korean cylindrical rice cakes rather than sliced soup rice cakes.
Chicken thigh stays moist during the skillet cooking and should reach 165 F (74 C).