Grind the sesame seeds in a coffee grinder until fine. Place the ground sesame seeds in a medium bowl and add the garlic, soy, sherry, honey, sesame oil, water, ginger, scallions, and red chili flakes; stir until thoroughly combined. Place the ribs in a large baking dish, pour the marinade over, and turn to coat. Cover and let marinate in the refrigerator for 2 to 3 hours.
Preheat the grill to medium-high. Remove the ribs from the marinade and grill on both sides until golden brown and crispy, about 15 minutes. Reduce the heat of the grill, close the cover, and grill until tender, about 15 minutes longer. Remove to a platter and sprinkle with sesame seeds and green onions.