Cut the onion into thin half rings and marinate:
for 1 onion, I take 2 tsp sugar + 1/5 tsp salt + 1.5 tsp 9% vinegar and pour it all with boiling water to the level of the onion in a bowl.
The taste of the marinade should be sweet and sour.
when the water cools down, the onion will be ready, flip it on a sieve to drain the liquid, or simply squeeze out excess moisture.