Generously season the short ribs with salt and freshly ground pepper all over. Whisk together the mustard, balsamic vinegar, soy sauce, sugar, the 1 tablespoon pepper, and the 1 teaspoon salt in a bowl to make the marinade; set aside.
Preheat a grill to medium. Grill the short ribs, covered, this way: Start with the bone side up, grilling for about 5 minutes. Flip the short ribs, so the bone side is now down (the bone will prevent the meat from burning), and brush the marinade on top. Grill about 15 more minutes, brushing with more marinade on top 3 more times. When the short ribs are ready, they’ll be richly browned and glossy. Test for doneness using “The ‘Nick-and-Peek’ Method”. Let the short ribs rest for 5 minutes and serve.