BBQ Beef Sandwiches

1 TBS vegetable oil
½ c chopped onion
1 26 oz can tomato soup
1/4 c water
2 TBS brown sugar
2 TBS vinegar
1 TBS worcestershire sauce
1 1/2 lb thinly sliced deli roast beef
8 hamburger buns

Heat the oil in a 4-quart saucepot over med heat. Add the onion and cook until tender, stirring occasionally. Stir the soup, water, brown sugar, vinegar and worcestershire in the saucepot and heat to a boil. Reduce the heat to low. Cook for 5 minutes. Add the beef to the saucepot and cook until the mixture is hot and bubbling, letting it simmer for at least 10-15 minutes. Divide the beef and sauce among the rolls.