Combine the condiments in a small bowl and set aside.
If using unseared bonito: Cut away any very dark parts of the bonito. Wash and pat dry. Spread out the fish and insert skewers--long metal ones are easiest to remove--parallel or fanning out to support the whole fillet. Sprinkle lightly with salt. Omit this step if you have bought seared bonito.
Quickly sear both sides of the fillets evenly over a very high heat. The outside of the bonito should turn white; the inside should look like rare steak. Omit this step if you have bought seared bonito.
Remove from the heat and plunge immediately into ice water. Gently remove the skewers by twisting them. Pat dry. Place the fillets on a cutting board, skin side up, and cut into 1/2-inch-thick slices.
Decorate a serving platter with the bonito slices, overlapping them in rows. Add a thick layer of the condiment mixture, patting down firmly, and drizzle liberally with ponzu sauce. Cover with plastic wrap and refrigerate for 3 to 4 hours.
Just before serving, remove from the refrigerator and take off the plastic wrap. Garnish if desired. Serve additional ponzu sauce in individual bowls, adding extra condiments if you like.
*I was able to buy already seared bonito at my local Japanese market here in the Bay area. It made for a faster preparation for this dish.
**If desired, make additional condiment mixture to mix into ponzu sauce at the table while dining.