Marinate the short ribs. Add the short ribs to a shallow baking dish. Combine the soy sauce, brown sugar, mirin, garlic, pear, ginger, scallions, sesame oil, gochujang paste, and gochugaru chili flakes in the carafe of a blender and blend until very smooth. Pour the marinade over the ribs, cover with plastic wrap, and refrigerate for at least 12 hours, up to 48 hours.