I love to use leftover meat this way because the thin slices, gently warmed, are so tasty tucked between slices of the lightly toasted baguette. The little bit of horseradish and mustard are really good here, too. Leave the meat sauce kind of thin and warm on the side with the sandwiches for dunking.
In a medium bowl, whisk together the mayonnaise, mustard, horseradish and vinegar. Taste for seasoning.2.
In a medium pan, simmer and reduce the leftover pan sauce with the beef stock over medium heat, 2-3 minutes. Thinly slice the leftover beef. Arrange and warm the beef gently in the sauce.3.
Spread the mayonnaise mixture on bottom halves of the baguette. Arrange half of the meat on each sandwich. And top with the other half of the bread. Serve with bowls of the meat sauce for dipping on the side.