In a large bowl or food processor, combine the soy sauce, brown sugar, water, grated Asian pear, minced garlic, sesame oil, ginger, black pepper, and half of the chopped green onions. Blend or mix until the sugar is completely dissolved.
Place the flanken-cut beef short ribs in a large baking dish or resealable plastic bag. Pour the marinade over the ribs, ensuring every piece is evenly coated. Cover and refrigerate for at least 4 hours, or preferably overnight.
Remove the ribs from the refrigerator 15 to 20 minutes before cooking to take the chill off. Preheat an outdoor grill or indoor grill pan to medium-high heat.
Remove the ribs from the marinade, shaking off any excess liquid. Grill the ribs for 3 to 4 minutes per side until the edges are caramelized, slightly charred, and the meat is cooked through.
Transfer the grilled ribs to a serving platter and garnish with the remaining chopped green onions. Serve immediately with steamed white rice, fresh lettuce leaves for wrapping, and a side of kimchi.