In a small cup, whisk together lemon juice, olive oil, mustard, and capers. Set aside.
Set out each individual plate or the platter on your work surface. Each plate will get about three or four slices of roast beef. The beef should be sliced onto sheets of paper. Turn paper over onto the plate and peel the paper off the beef slices. Fold beef into loose accordion like folds. Repeat among all the plates.
In a separate bowl, toss arugula with the dressing. Arrange dressed greens in a pile over the roast beef in the center of each plate, about one cup per plate. Place about five curls of cheese on each plate on top of the arugula. Arrange tomatoes, cut side up, around the rim of each plate. Serve immediately.