You need a casserole dish with a lid that will go from stove top to oven.
Preheat the oven to 350°F (180°C or 170°C for a fan oven).
Heat the oil in the casserole dish over medium heat on the stove top.
Sauté the leeks for two or three minutes, getting them well coated in the oil.
Tumble in the carrots and parsnips and cook for moment or two.
Stir in the mushrooms.
Rinse and sort the pearl barley.
Add the pearl barley to the vegetables and stir well to coat it in the oil.
Stir in 3½ cups of the stock along with the basil, pepper and nutmeg.
Cover the casserole and put it in the oven for 30 minutes.
Remove the casserole from the oven and stir the stew, adding a bit more stock if the barley has absorbed it all.
Return the casserole to the oven for another 20 - 30 minutes.
Remove the casserole from the oven and stir again, checking to see if the barley and vegetables are tender.
You can add a bit more stock to loosen the stew if desired, but you want it to be quite hearty.
Carefully taste for seasoning (it will be very hot) and adding salt and pepper if necessary.
Serve in warmed bowls.