BBQ Beef Sandwiches

6 lb. beef roast (sirloin tip works well)
1 onion, chopped
2 T. butter
2 c. ketchup
1 c. packed brown sugar
2 T. Worcestershire sauce
1/2 c. barbecue sauce
1 dozen large, soft rolls

Cook roast until desired doneness, either in the oven or in a slow cooker. Cool for about 14 minutes then cut the roast into thin, sandwich-sized slices. Return sliced meat to pan with drippings. Sauté the chopped onion in butter. In a large saucepan, combine ketchup, brown sugar, Worcestershire sauce, barbecue sauce, and sautéed onion. Heat through. Pour desired amount over the beef. Fill the rolls just before serving. Makes 12 sandwiches. This is a great dish for potlucks or picnics.