Peel and finely chop the onion. Season the vinegar with salt, pepper and sugar, fold in olive oil. Stir in diced onion. Fold the vinaigrette into the salad mix. Stir yoghurt until smooth.
Stir in herbs and season with salt, pepper and sugar. Heat oil in a pan. Fry the fish cakes in it at medium heat on each side for about 4 minutes. Arrange 2 fish cakes with some yoghurt and lamb's lettuce on plates.
It goes well with baguette bread.