Finely dice the onion and drizzle with the vinegar (marinate for about 15 minutes - this way the onion loses its sharpness and becomes even nicer red).
Pluck the tarragon from the stems, wash and spin with the rocket.
Halve the grapes and add to the salad.
Put the yogurt in a bowl and season with salt, pepper, olive oil and the red onion vinegar.
Add the onion to the salad and serve with the yogurt.