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Marinate overnight; keep marinade salty in mind when salting initially.
Prep the ribs the day before cooking by seasoning lightly with salt and letting sit 30 minutes. Remember the marinade is salty so use about half the usual salt.
Whisk together soy sauce, brown sugar, mirin, sesame oil, grated ginger, grated garlic, and cayenne.
Place the meat in a resealable plastic bag, pour in the marinade, seal, and refrigerate overnight, squishing to coat the meat.
A few hours before cooking, let ribs come to room temperature.
Adjust oven rack to middle position and preheat oven to 400°F.
Preheat a large cast-iron skillet over medium-high heat, then reduce to medium-low and add enough oil to coat the bottom.
Working in batches to avoid crowding, brown short ribs on all meaty sides, 4 to 5 minutes per side as needed to achieve a deep brown. Add more oil between batches if necessary.
Transfer ribs bone-side down to a large Dutch oven or deep roasting dish and pour the reserved marinade over the meat so ribs are in a single layer.
Add enough dashi to come about 3/4 inch up the sides of the ribs. Cover with a lid or with parchment and aluminum foil.
Slide into the oven and cook until the liquid simmers, about 30 minutes, then reduce oven temperature to 325°F and braise until meat is very tender and falling off the bone, about 3 to 4 hours.
Uncover and increase oven to 450°F. Brush ribs with braising juices and cook until shiny and browned, about 8 to 12 minutes.
Serve warm garnished with cilantro and slivered scallions. Cover and refrigerate leftovers for up to 3 days.