Heat butter in a heavy pot over medium-high heat. Saute the onions for a couple of minutes, or until starting to get brown.
Set chuck roast on top of the onions.Add all remaining ingredients. Cover pot and simmer (very low heat) on the stove for 6 hours, or until beef is fork-tender and falling apart. **If meat is not yet tender, return to oven for 30 minute intervals till it’s tender!** Shred meat with two forks until all large chunks are gone. Serve immediately or continue to simmer for 30 minutes to 1 hour.
*Mixture can be refrigerated overnight. Remove hardened fat from top of pan before reheating.
Serve on top of toasted, buttered deli rolls. Top with cheese and place under the broiler if desired.