Mix the dressing of salt, sugar, pepper, mustard, vinegar and oil together (e.g. shake it together in a jam jar), pour over the salad (or mix with the salad at the same time), add the onion rings and the fish fillets cut into pieces to distribute.
If you want, you can also add 2 hard-boiled eggs, cut into wedges, to the salad - or with pomegranate seeds (half a pomegranate is enough).
The tastes of salad dressings are very different. I use a lot of salt, sugar and vinegar - it creates a strong sweet and sour note. If you don`t like that, take less sugar and vinegar. With the oil you can also take more according to your individual taste or add a little pumpkin seed oil to the dressing (goes particularly well with a raspberry vinegar).