The key to getting the perfect sliced roast beef is to sear it on all sides to lock in flavor. Then, the roast is transferred to the oven, where it cooks for 30 minutes.

At Mr. Beef on Orleans, which has been around since 1979, they roast beef in Italian seasonings, slice it thinly, and let it get happy in its own juice while waiting to be eaten.

Last month, Dan Michaels, an owner of Gino’s East of Chicago in Los Angeles, watched as orders for Italian beef — the classic Chicago sandwich of thinly sliced roast beef and tangy giardiniera ...