Rinse the short ribs under cold water. Place in a large bowl of cold water and soak for 30 minutes to draw out excess blood. Drain, rinse again, and pat dry with paper towels.
Combine soy sauce, sesame oil, brown sugar, mirin, rice vinegar, grated Asian pear, grated onion, minced garlic, grated ginger, water, and black pepper in a medium bowl. Whisk until the sugar is dissolved.
Preheat a charcoal or gas grill to high heat (450-500°F / 230-260°C). Clean and oil the grates well — the sugar in the marinade will stick if grates are not properly oiled.
Remove ribs from the marinade, letting excess drip off. Place on the grill in a single layer.
Grill for 3-4 minutes per side, until nicely charred and caramelized. The thin cut cooks quickly — do not overcook or the meat will become tough.
The Asian pear is key. It acts as a natural tenderizer due to its enzymes and adds subtle sweetness. If unavailable, substitute ½ Bosc pear plus 1 tbsp (15 ml) apple juice.
Flanken cut matters. Ask your butcher for short ribs cut across the bone, about ¼ inch (6 mm) thick. Korean grocery stores almost always carry this cut.
Don't marinate longer than 24 hours. The pear enzymes can make the meat mushy.
No grill? Use a cast-iron grill pan or broiler set to high, about 4 inches (10 cm) from the heat source, 3-4 minutes per side.