More interesting, more flavorful, and far less fat‐laden than traditional coleslaw.
Whisk together the mustard and vinegar in a small bowl; add the sugar, then add the oil a little at a time, whisking all the while.
Combine the cabbage, peppers, and scallions, and toss with the dressing. Season with salt and pepper and refrigerate until ready to serve (it's best to let this rest for an hour or so before serving to allow the flavors to mellow; you can let it sit longer, up to 24 hours, if you like).
Just before serving, toss with parsley.