Toast the sesame seeds in a dry skillet over medium heat, stirring, until golden and fragrant, 3–5 minutes. Transfer to a plate to cool.
In a large bowl, combine the napa cabbage, red cabbage, carrots, red bell pepper, scallions, and cilantro.
In a small bowl or jar, whisk together the rice vinegar, soy sauce, sugar, ginger, garlic, neutral oil, and toasted sesame oil until emulsified.
Pour the dressing over the vegetables and toss thoroughly to coat. Let stand 5–10 minutes until the cabbage slightly wilts but remains crisp.
Sprinkle in the toasted sesame seeds and toss again. Taste and adjust with a splash of soy sauce or vinegar if needed, then serve immediately or chill up to 2 hours and toss before serving.