In a large bowl, combine the green cabbage, red cabbage, carrots, green onions, cilantro, and toasted sesame seeds. Toss well to mix the vegetables evenly.
In a separate small bowl, whisk together the soy sauce, rice vinegar, sesame oil, honey, grated ginger, minced garlic, and red pepper flakes if using. Mix until well combined.
Pour the dressing over the cabbage mixture and toss until all the vegetables are coated evenly.
Let the slaw sit for at least 15 minutes before serving to allow the flavors to meld. For best results, refrigerate for 30 minutes.
Serve chilled or at room temperature as a side dish or light meal.