Place chuck roast and onions in a 6-quart or larger slow cooker.
In a medium bowl, whisk together Coca Cola and next 8 ingredients plus crushed red pepper flakes if using.Measure out 1/2 cup and place in refrigerator.Pour remaining mixture over chuck roast.
Cover and cook on LOW for 8 to 10 hours or on HIGH for 5 to 6 hours. Roast should be very tender and easy to shred.
Drain most of the liquid out of the slow cooker. Shred the chuck roast.
In a bowl, stir together the ketchup, melted butter and the refrigerated 1/2 cup of coca cola mixture. Microwave for 20 seconds to warm. Pour over shredded chuck roast. Mix it in.Cover slow cooker and cook on LOW for 15 minutes. Serve.