Saute onion and garlic with oil in the cooking pot until transparent.
Season the roast with salt and pepper and then dredge (lightly) in flour.
Brown roast (on top of the onions and garlic) in the crockpot on both sides (10 minutes).
Transfer to the slow cooker, add the stewed tomatoes, water and worcestershire sauce.
Cook on setting 4 (of 5) for about 5 hours.
When done, puree the remaining tomatoes, juices and onions, return to pan, add thickener for gravy.