Season the beef round with salt and pepper. Put it in a Dutch oven or kettle filled about half way up with water. Add the onion, celery, garlic, carrots, bay leaves, thyme, marjoram, and peppercorns. Roast it, uncovered, at 350 degrees for four to five hours, turning the roast and adding water every hour or so. The water level should slowly drop, but don't let it get less than about two inches deep. The beef is ready when a meat thermometer pushed into the center of the beef reads 160 degrees.
Remove the roast from the pot and place in a pan that will catch all the juices that come out as it cools. If you're cooking a day ahead (recommended), wrap the beef and refrigerate it as soon as it's cooled to room temperature. In any case, wait at least an hour before slicing.
Strain the solids from the stock in the pot. Bring the stock to a simmer. After removing excess fat, add all the juices that come from the roast, as well as the crumbs of beef that fall off as you slice it. Skim off the fat that rises to the surface. Cook to a light gravy consistency. (This also benefits from being made a day ahead, and cooling in the refrigerator.)
When you're ready to make sandwiches, bring the gravy to a simmer and whisk in the flour (but only if the gravy appears to need thickening). Add salt, pepper and Worcestershire to taste. (It's a common practice in New Orleans to add Kitchen Bouquet to darken the sauce, but I never do.)
Slice the roast beef as thin as possible and put as much as you want on fresh French bread with lettuce, tomatoes, mayonnaise, and dill pickles. Spoon on all the gravy the sandwich can hold. After assembling the sandwich, put the whole thing into a 400-degree oven for about a minute to toast the bread.
