Brush any dirt from the mushrooms and slice finely. Peel and finely slice the onions.
Heat the olive oil over a low-medium flame in a large pan, and add the mushrooms and onions. Saute for 8-10 minutes, stirring occasionally, until the vegetables soften.
Add the pearl barley, porcini mushrooms (if using), pepper and bay leaf, and 1 litre of hot water.
Stir well, cover the pan, and reduce the heat to the minimum required to sustain the barest simmer. Cook the soup for 1½-2 hours, stirring occasionally.
When ready to serve, remove the soup from the heat and allow to cool for a minute or two. Cream the miso in a small bowl with 1½-2 tbsp of hot, not boiling, water, then stir this mixture into the soup.
Serve at once, garnished with chopped fresh chives or parsley.
