In a large mixing bowl, or in the bowl of an electric mixer, combine all of the ingredients except the topping, mixing to form a shaggy dough.
Knead the dough, by hand or by machine, for 10 minutes. Allow it to rest for 10 minutes, then knead for an additional 10 minutes, until it's smooth and supple.
Place the dough in a lightly greased bowl, cover the bowl, and allow it to rest for 1 hour; it'll become quite puffy, though it may not double in bulk. You may also use your bread machine, set on the Dough cycle, to prepare the dough to this point.
Divide the dough into six 3 1/4-ounce (92g) pieces, and shape them into smooth, slightly flattened rounds. Place them on a baking sheet, covered, to rise for 1 hour, or until they're good and puffy.
While the rolls are rising, preheat the oven to 425°F.
Brush the rolls with the beaten egg white, and sprinkle lightly (or not) with the topping. Bake them for 15 to 18 minutes, or until they're golden brown.
Remove from the oven and cool on a wire rack; they're best served the same day they're made, though they do reheat nicely. Freeze for up to 1 month.
TO MAKE A BEEF ON WECK SANDWICH
Warm the au jus sauce in a saucepan over medium heat. Add the roast beef and let it warm up at the same time.
Carefully dip the top bun in the au jus. Not a dunk or anything, just enough to moisten the part that will come into contact with the roast beef.
Pile the beef on the bottom bun, slightly higher than you think you should. Beef on Weck is all about go big or go home.
Serve the sliced beef on the rolls with the au jus and horseradish on the side.