Dippers can't help themselves: To them, a sandwich always tastes better dunked, be it in a bowl of hot soup, a puddle of salsa or a smear of ketchup on their plate. Miss Manners might not approve, but they go on dipping just the same.

When roast beef leftovers are in your fridge, you'd be wise to join them.

French dip sandwiches, the lunch-counter favorite, originated in Los Angeles nearly a century ago (the name comes from the French roll it's served on). The idea is simple: Stuff a crusty roll with sliced roast beef or beef brisket and serve it "au jus," literally, doused with hot pan juices.

At the two landmark restaurants that claim to have invented it, Philippe the Original and Cole's P.E. Buffet, a French dip comes "wet," meaning the roll is dipped in hot pan juices before the sandwich is assembled. Almost everywhere else it's served dry with a cup of hot beef broth, so you can happily dunk your own as you nibble away.

Regardless of how you choose to serve it at home, a French dip is only as good as the components that go into it. Here's our advice for getting it right.

The meat

The better the beef, the better your sandwich. If you're serving rib-eye roast or tenderloin, all you need to remember the day after is to slice it thinly when it's cold and trim off as much fat as possible. A handy substitute is roast beef from a deli, but make sure to buy top quality, and taste it before plunking down your money.

The roll

Only crusty bread will do for this sandwich, so look for something sturdy that can soak up the broth. A baguette or a torpedo-shaped French roll is our first choice. Anything that resembles a hamburger bun will be too soft and turn to glop.

The broth

The "au jus" is the secret of a great French dip, and it will only be as good as your stock. A rich, beefy and pleasantly salty flavor is what you're after, so it's not just warm and moist but a distinct part of the sandwich.

We recommend making au jus from scratch if you have the time, but do it a day ahead so the flavors meld and you can skim the fat off the top. Instant au jus mixes are fine if you're in a hurry, but be picky about which you choose. Of the half-dozen supermarket brands we tested, Crescent and Lawry's were the best, with Johnny's powdered mix (not Johnny's sauce) close behind. Others were too salty or had off flavors.

The assembly

The rest is easy. Lightly toast the rolls, and if you like a warm sandwich, heat the roast beef slices in the simmering broth, scoop them out with a slotted spoon and divide evenly among the rolls. The hot broth will cook the beef further, so don't use this method if the meat is already as well-done as you want it.

Either way, fill mugs or small bowls with piping-hot au jus and bring to the table with the sandwiches.

The dippers will be waiting.