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.