Thinly shred the green and red cabbage and place in a large mixing bowl.
Peel and julienne the carrots; add them to the cabbage.
Sprinkle the salt over the vegetables and toss; let sit 5 minutes to draw out moisture.
In a separate bowl, whisk together apple cider vinegar, sugar, Dijon mustard and olive oil until the sugar dissolves.
Add celery seed and black pepper to the dressing; taste and adjust sweetness or acidity.
Pour the dressing over the cabbage-carrot mix and toss thoroughly to coat.
Add sliced green onion and gently fold to combine.
Let the slaw rest 15–30 minutes in the fridge to soften slightly and allow flavors to meld.
Toss again before serving to redistribute any settled dressing.
Garnish with chopped parsley or dill if desired and serve chilled or at cool room temperature.