Special equipment: a mandoline
Combine the green and red cabbage in a large bowl. Add the parsley, vinegar, mustard, sugar, peppers, onions and some salt and pepper and mix to combine. Let sit for 30 minutes.
Drain the excess liquid from the bowl. Transfer the slaw to a piece of cheesecloth and lightly squeeze to remove more liquid. Return the slaw to the bowl and add a little more vinegar if necessary.