Place the scallions, soy sauce, brown sugar, ginger, garlic, vinegar, sesame oil, black pepper and cayenne pepper in a food processor or blender and blend until smooth.
Place the short ribs in a container, pour over the marinade and turn to coat well. Cover the short ribs and refrigerate from 12 to 24 hours.
Just before grilling, toast the sesame seeds, if using, by heating a small skillet over medium high heat, then adding the seeds. Toss and stir for a few minutes until they become deeper golden in color, but watch carefully as they can burn quickly. Transfer to a small plate.
Preheat the grill to medium high. Remove the short ribs from the marinade. Grill for about 4 minutes on each side, until the outside is caramelized and the middle is medium-rare. Allow the meat to sit for 5 minutes before slicing across the grain and serving with the hot rice. Or, if you prefer (and do heavily consider this), thinly slice the meat and serve it with any or all of the suggested accompaniments. Let each diner wrap up some meat and rice with whatever extras they want, and make it an interactive dinner.
Notes
Short Ribs in the Broiler
You can also broil these instead of grilling during months where you are cozying up to your stove instead of your grill.