In the bowl of a food processor, finely chop the pear, scallions, and garlic. Add the water, soy sauce, sugar, and the rice vinegar. Process until well combined.
Put the ribs in a large bowl and pour in the marinade. Mix well to coat the ribs evenly. Cover and refrigerate for 2 to 4 hours.
Prepare the grill for direct cooking over high heat, [450°F to 550°F (230°C to 290°C)].
Brush the cooking grates clean. Lift the ribs from the marinade 1 at a time, letting the excess liquid and solid bits fall back into the bowl before laying them on the grill. Discard the marinade.
Grill the ribs over direct high heat, with the lid open, until they are nicely charred on both sides and cooked to a medium or medium-rare doneness, 3 to 5 minutes, turning occasionally. Remove from the grill and sprinkle with the sesame seeds.