In a blender, combine low sodium soy sauce, brown sugar, water, mirin, sesame oil, minced garlic, grated ginger, chopped onion, salt, and freshly ground black pepper. Blend until smooth and fully integrated, about 1-2 minutes.

Place the thinly sliced beef short ribs in a glass dish, pour the marinade over them, cover with plastic wrap, and refrigerate for at least 1 hour or overnight.

Preheat your grill to medium-high heat. Remove the short ribs from the marinade and discard the excess. Grill for approximately 3-4 minutes on each side until caramelized.

Transfer the marinated short ribs along with the marinade into your crockpot. Set it to high and cook for approximately 4 hours until tender.

If using an air fryer, arrange the marinated short ribs in a single layer and cook at 375°F for about 5-7 minutes, flipping halfway.