Toast sesame seeds in an 8- to 10-inch frying pan over medium heat until golden, about 2 minutes, shaking pan frequently.
Crush coarsely with a mortar and pestle or whirl briefly in a blender.
Place ribs, bone side down, on a cutting board.
Make a series of parallel cuts 1/2 inch apart, cutting halfway to bone each time.
Then make another series of parallel cuts at right angles to the first set; make cuts 1/2 inch apart and ¥1 inch deep.
In a bowl, combine sesame seeds, soy, sugar, garlic, ginger, oil, and onions.
Add meat; turn to coat.
Cover and refrigerate for at least 4 hours or up to 8 hours.
Lift ribs from marinade, drain briefly, and place on a grill 4 to 6 inches above a solid bed of hot coals (you should be able to hold your hand at grill level for no more than 2 to 3 seconds).
Cook until browned on all sides and done to your liking (cut to test), 15 to 20 minutes for medium-ran.