Fill a pan with boiling water from the kettle then crumble in ¾ of the stock cube and stir to dissolve. Cook the pearl barley for 45 minutes until tender.
Heat the oil in a frying pan and cook the onion for 8-10 minutes then add the garlic and cook for 1 minute. Trim the stalks off the mushrooms with a small knife, roughly chop and add to the pan to cook for 2 minutes then take off the heat.
Preheat the oven to 200c/180c fan/gas mark 6.
Add the cooked pearl barley and the herbs to the onion mix then crumble in the remaining stock cube and season to taste.
Carefully fill the mushrooms with the pearl barley and bake on a tray in the oven for 20 minutes.
Meanwhile steam the carrots and swiss chard until just cooked and toss with the lemon juice and some seasoning.
Serve the mushrooms alongside the vegetables and any juices from the baking tray drizzled over.