Fry the mushrooms in a spray of oil for 3 minutes until golden. Season and scoop out the pan. Fry the onion in another spray of olive oil for 2 minutes before adding the thyme and garlic, and fry for 1 minute. Add the barley, mix with the onions and gradually add the stock, stirring frequently. Simmer for 25-30 minutes until the barley is tender, adding more stock if needed. When the stock has been absorbed, stir in the spinach, cheese and mushrooms. Season and cook for 2 more minutes until the spinach has wilted.