Cut onions into half lengthwise, slice and separate into rings. Trim all possible fat from roast. Sprinkle with powdered garlic, salt and pepper. Place 1/2 of onions in crockpot, then roast, then top with rest of onions. Pour coke over all. Cover and cook on LOW 10-12 hours, or overnight until roast can be easily shredded with a fork. Remove pork from crockpot. Drain onions from juice and dicard. Shred meat into a large bowl, removing all possivle fat. Return to crockpot. Add barbecue sauce, mix will and cook on Low 5-6 hours, adding sauce if desired. Serve on buns.
Freezing: Freeze in meal-size containers. Re-heat in microwave.