In a large mixing bowl, combine the shredded green cabbage, shredded red cabbage, grated carrot, and sliced green onions. Toss them together gently to mix evenly.

Use a sharp knife and cutting board to finely shred the cabbages and grate the carrot. This ensures that the vegetables are tender and easy to eat.

In a separate bowl, whisk together the mayonnaise, apple cider vinegar, sugar, Dijon mustard, celery seed, salt, and black pepper until smooth. This dressing will coat the vegetables and add flavor.

Taste the dressing and adjust the seasoning if necessary, adding more salt or pepper to suit your preference.

Pour the dressing over the vegetable mixture and toss until all the vegetables are evenly coated. This step is crucial as it ensures every bite is flavorful.

Cover the coleslaw and let it sit in the refrigerator for at least 30 minutes before serving. This allows the flavors to meld together and the cabbage to soften slightly.

Serve chilled as a side dish or topping for sandwiches and burgers.