Pour stock in a saucepan over high heat and bring to the boil. Reduce heat to very low and hold at a gentle simmer. Heat oil in a large heavy-based saucepan over medium heat. Add the sliced mushrooms and cook, stirring, for 5 minutes or until soft. Stir in the barley.
Add a ladleful (about 125mls or ½ cup) of simmering stock to the barley mixture and cook, stirring constantly, until liquid is absorbed. Add stock, a ladleful at a time, stirring constantly and allowing the liquid to be almost absorbed before adding the next ladleful, for 20-25 minutes or until barley is tender
Meanwhile, melt the half butter in a large frying pan over high heat. Add the flat mushrooms and cook for 3-4 minutes each side until tender.
Add the remaining butter, parmesan and spinach to the barley mixture and season to taste. Spoon into the centre of serving bowls, top with mushroom, extra parmesan and rocket or watercress and serve.