Fry the onions in a large saucepan with the butter. Add the leek, carrots and celery, stir everything and simmer in a closed pot for 5 minutes. Add the pearl barley, the stock, salt and pepper, stir and cook covered over a mild heat for 40 minutes.
Then stir in the potatoes and simmer for another 20 minutes. Pour in a little more broth if necessary. Finally, season again to taste.