Place roast, onions, celery, garlic and water in a large pot. Bring to a boil then cover and turn down to low heat and simmer the roast for at least 4 hours (the roast should fall apart when you press with a fork).
Remove from liquid, saving about 1 cup of liquid, and cool slightly. Shred roast and add to the remaining liquid. Add all remaining ingredients and cook uncovered for 1 hour.