Ideally, make this an hour before, and let it sit in the refrigerator to let the flavors mingle and the cabbage soften slightly. The veg can be shredded a day or two before and kept in the fridge, and the dressing will store in the fridge for weeks.
Quarter the cabbage through the core, and then cut out the core. Cut each quarter crosswise in half and finely shred. Place the shredded cabbage in a very large bowl.
Add the grated carrot and parsley to the cabbage and toss to mix
In a separate bowl, stir the mayonnaise, vinegar, mustard, celery seeds, salt, and pepper together. Taste for acidity and seasoning, then adjust as desired. If the dressing tastes too tart, stir in the optional honey.
Pour two-thirds of the dressing over the cabbage and carrot then mix well.
If the coleslaw seems dry, add a little more of the dressing.