Cut leftover sliced beef into wide slivers, and cut any leftover potatoes and vegetables into bite-size pieces. Make a fairly tart vinaigrette with Dijon mustard, red wine vinegar, salt and pepper, and olive oil. In a large bowl, combine the meat, potatoes, and vegetables with enough of the vinaigrette to moisten, adding a few capers and some chopped fresh parsley. In a second bowl, toss a mix of bite-size salad greens with enough vinaigrette to coat.
Arrange the greens on a serving platter or on individual serving plates. Top with the meat-vegetable mixture and serve.