Peel and thinly slice the onions, place them in a lidded container, add sugar and salt, cover and shake the container up and down about 10 times, then let it sit for 3-5 minutes.

Prepare a bowl with about half a bag of store-bought ice cubes and add 1/2 cup (about 100 ml) of water. Drain the onions and add them to the bowl (I like to squeeze them by hand to test the texture).

Soaking the onions in ice water removes the pungency and adds crispness, so soak them for as long as you like (I usually do 10-15 minutes).

Drain the onions from the ice water, add the tuna (if the can is oil-packed, you can keep about 1 tablespoon of the oil), and 1 tablespoon of fish sauce. Adjust based on the saltiness of the tuna. If desired, add black peppercorns, white pepper powder, sesame oil, but try not to add everything to avoid overwhelming the flavors.

Once seasoned, add half a packet of bonito flakes and mix briefly. When serving, sprinkle the remaining half packet on top.