Pop the porcini in a heatproof jug. Crumble in the stock cube. Stir in 1 ltr boiling water. Set aside to soak for 15 mins. Peel and finely chop the onion. Warm 2 tsp oil in a pan over a very low heat. Add the onion. Season well with salt and pepper. Cover with a square of greaseproof paper and a lid. Gently cook for 10 mins till soft but not coloured.
Finely slice the mushrooms. Remove the paper from the onion pan. Add the mushrooms. Stir in the pearl barley. Add the porcini mushrooms and their stock. Chuck in the sprigs of thyme. Cover and bring to the boil. Turn the heat down and simmer for 20 mins till the barley is tender.
Lift the thyme sprigs out of the soup. Taste it and add more salt and pepper if you think it needs it. Stir in the spinach so it just wilts. Ladle into warm bowls and serve straight away.