Whisk the mayonnaise, mustard, sugar, vinegar, buttermilk, celery seeds, salt and pepper in a small bowl until well combined. Taste and adjust seasoning for sweet, salt, vinegar, as desired.
Combine the cabbages, carrot, bell pepper, onion, and parsley in a large mixing bowl.
Toss with enough dressing to evenly coat the slaw. Cover and refrigerate for 1 hour to chill and meld flavors.
Notes
TIPS & VARIATIONS:
Coleslaw can be prepped up to 4 days in advance without the dressing. Once the dressing is added, enjoy within 1-2 days for best results.
To save a little time, shredded cabbage or broccoli slaw can be purchased already shredded. If using broccoli slaw add a colorful aspect with chopped radicchio, sometimes known as Italian chicory, in place of the red cabbage.
Coleslaw Substitutions or Mix-in Options:
Apple
Fennel
Radishes
Celery
Grated broccoli or cauliflower
Shaved Brussels sprouts
Jicama
Leeks
Pineapple bits
Chile peppers
Shallots
Dried fruits like mango, cranberries, or raisins
Nuts and seeds like sliced almonds, pepitas, or sunflower seeds