With the point of a knife, punch slits into the top of the beef. Insert the quartered cloves of garlic.
Cover the bottom of a large Dutch oven with the sliced onions. Place the beef atop the onions. Scatter the salt, Worcestershire, and barbecue sauce over the beef.
Pour a half-gallon of water around (not over) the beef. Put the pan into the oven, uncovered, at 350 degrees, and roast until the top of the beef turns noticeably brown--about 45 minutes.
Turn the beef 180 degrees. Add enough water that the beef seems nearly weightless, as if it were about to float. Continue cooking, turning the beef about 90 degrees every 45 minutes, for three hours.
Check the doneness of the beef. It should have a blush of pink in the center--about 160 on a meat thermometer. If it's not there, keep going, but know that the speed of the cooking will increase, so check it more often.
Remove the beef from the dutch oven onto a pan, to collect the juices. Pour the stock from the dutch oven into a large saucepan. Add the juices that come out of the roast. Add the bay leaves and bring the pan contents to a simmer. Cook until reduced to about one-quarter of what you started with--about six to eight cups of liquid. Strain.
Optional: Make a medium-dark roux with a half-cup each of flour and vegetable oil. When done, add about a half-cup of reduced stock to the roux and whisk until smooth. use this mixture to thicken the stock to gravy consistency.
Adjust the gravy with salt and pepper. Slice the roast beef as thinly as you can, and dip into the gravy before putting the beef onto one half of a mayonnaise-spread French bread sliced end to end. Spread the other side with mayonnaise and add the lettuce, tomatoes, and pickles. Put the whole sandwich into a preheated 450-degree oven until toasted. Eat with a root beer or a not-so-root beer.