Combine the slaw mix, bell pepper and onion in a large resealable container. Toss with the salt and pepper. Cover and refrigerate for 1 hour.
Meanwhile, combine the vinegar, sugar, mustard powder, and celery seeds in a small saucepan and bring to a boil. Stir until the sugar is dissolved and remove from the heat and let cool until the cabbage mixture has been in the fridge for 1 hour.
Remove cabbage from fridge and pour sauce over it. Add the oil, green onions, and parsley and toss to combine.
Cover and return to refrigerator for 2-3 hours before serving.