Cut the thick center vein out of the collard green leaves then stack several leaves together, roll into a cylinder, hold together and thinly slice to make narrow strips. Repeat with remaining leaves and transfer to a large bowl.
Stir in carrots, bell pepper, and onion. In a small sauce pan, whisk together vinegar, sugar, oil, mustard, celery seed, salt and pepper and bring to a boil. Pour the hot dressing over the vegetables and toss to coat well. Season with salt and pepper if desired.
Transfer to a large zip lock bag and close…sealing out any air. Refrigerate overnight.
