In a small bowl, mix together 1 teaspoon salt and the sugar; add the lemon slices and toss. Let stand for 1 hour.
Meanwhile, in a large bowl, whisk together the olive oil, mustard and vinegar; transfer one-third of the marinade to a medium bowl. Add the beef to the large bowl, and the tomatoes and scallions to the medium bowl; toss. Refrigerate for at least 1 hour.
Preheat a grill to medium-high. Thread each of eight 10-inch skewers with a beef cube, lemon slice, tomato wedge, another beef cube, a scallion piece and another tomato wedge.
Brush the grill with olive oil. Season the kebabs with salt and pepper and grill, turning occasionally, for 8 to 10 minutes for medium-rare.