Peel the onions and cut them into rings that are not too thin (then weigh them) - these can be up to 1 cm wide. Then place them in a heatproof bowl.
Bring the vinegar, sugar and salt to a boil in a saucepan. Stir constantly until the sugar has completely dissolved. While hot, pour over the onions and stir briefly. Let it stand like this until it has cooled down, the onions will then turn pink in the meantime.
Then put everything in a larger screw-top jar and put it in the refrigerator, where it can be kept for up to two months without any problems. The onions are very fine with a snack or pan-fried meat.