Wash the linen thoroughly. Cut the onion into small cubes.
Let the oil get hot in a large deep pan and fry the onion until translucent. Then put the Postelein, which is still dripping wet, into the pan. After a short time, the Postelein collapses. Scatter the vegetable stock over it and, if necessary, pour a little water over it. Steam the whole thing for 2-3 minutes.
Pour off excess liquid. Gently squeeze the vegetables (goes well with a wooden spoon in a sieve). Roughly chop with a large knife and put back in the pan. Stir in the crème fraîche and season with salt and pepper.
The Postelein vegetables taste similar to leaf spinach and can be used as a nice side dish or as a filling for pasta.