Tip the prepared vegetables into a heavy-based casserole that comes with a lid, then add the barley and chicken thighs.Pour the stock into a jug, stir in the mustard, then pour this over the contents of the pan. Bring to the boil - and this is when better-behaved cooks would tell you to skim off the frothy bits that rise to the top but, frankly, I'm too lazy to - then turn down the heat, partially cover and let simmer for 1 hour, though check every now and again to make sure it's not bubbling away too much or dolefully not enough, giving a stir as you do so. If it looks as if it's boiling dry, then pour in a little boiling water.Once the hour's up, the barley, vegetables and chicken should be tender and the juices all but absorbed. Not that a little soupiness would be the end of the world. Remove from the heat and let stand with the lid off for 10 minutes.If you haven't used a skinless thighs, remove and discard the chicken skin. Using a couple of forks, pull the meat off the bones, and discard the bones (my particular treat is to chew the cartilage off them before chucking them away, but it's not to everyone's taste). Then throw in some of the chopped parsley and stir it through the stew, and put the rest on the table for people to sprinkle over their own bowls as they eat. It probably goes without saying that if you're feeding small children, predisposed to be pernickety about Green Bits, then you would be ill-advised to stir any parsley into the stew.