Cook the pearl barley according to package instructions, drain and set aside to cool.
Cut the squash in half lengthways. Scoop out the seeds from both halves and discard them. Continue to scrape out the centre of the squash, to make room for the stuffing.
Score the flesh of the 2 squash halves in a criss-cross pattern, trying not to pierce the skin, rub with 1 tablespoon oil and season.
Heat oven to 180˚C, 160˚C fan, gas mark 4.
In a frying pan, heat 2 tablespoons of oil and fry the onion, leek and mushrooms for 5-8 minutes until soft. Stir in the garlic and cook for another minute.
Add the pearl barley, nuts, spices, thyme, dried fruit and lemon zest. Season and cook for 1-2 minutes.
Spoon the filling into the cavities of the squash halves, keep any leftover stuffing.
Secure the halves together with kitchen twine good and tight as the squash will shrink a little as it cooks.
Rub the outside with the remaining oil and season well.
Add the squash to the Magic Chicken Roasting Bag, secure the bag with a nylon tie and place the bag on a tray.
Before cooking, trim any excess bag. Use a scissors to make slits in the top of the bag.
Cook for 45 minutes- 1 hour or until pierced with a knife there is a little resistance and the flesh feels soft.
Cut the bag open and carefully lift the squash onto a serving plate/platter.
For the sauce, mix the tahini, lemon juice, honey and 1-2 teaspoons of water to make a dressing.
In a pan, warm any leftover stuffing.
Carve the butternut squash. Serve with mixed leaf salad, tahini sauce and extra stuffing.