Select the herbs (see chef's note for list of traditional herbs), wash, and spin dry. Chop finely with a knife or in a food processor.
Add the finely diced onion, juice of 1/2 lemon, mustard, quark, sour cream and yogurt to the herbs in a food processor or with the herbs in a bowl and mix well. Season to taste with salt and pepper.
Serve the green sauce cold. Classically with boiled and halved eggs and boiled potatoes or breaded schnitzel, which is thereby upgraded to "Frankfurter Schnitzel". However, the sauce also goes well with fried or smoked salmon or cold roast meat.