Cook the pearl barley following pack instructions, crumbling the stock cube into the boiling water for extra flavour. Drain and set aside to cool.
Meanwhile, toast the pine nuts and almonds in a dry pan until nutty brown. Tip into a bowl and set aside. Use the same pan to toast the fennel and coriander seeds for a couple of mins, until aromatic. Grind the spices using a pestle and mortar.
Cut the squash in half lengthways through the centre. Scoop out and discard the seeds and any fibrous bits. Keep scraping to carve out a channel down the centre of both squash halves for stuffing, you may need to score the flesh with a knife first if your squash is very firm. The squash shell should be about 1.5cm thick all over when you’ve finished. Roughly chop any squash you remove and set aside. Score the flesh of the 2 squash halves in a criss-cross pattern, trying not to pierce the skin, rub with 1 tbsp oil and season well.
Heat oven to 180C/160C fan/gas 4. Heat the remaining 2 tbsp oil in a pan and cook the onion, chopped squash and leek for 8-10 mins until soft. Stir in the garlic and cook for another min. Add the pearl barley, nuts, spices, feta, herbs, dried fruit and lemon zest. Season well and cook for a min or 2 more, then pack the filling into the cavities in the 2 squash halves, keep any leftover stuffing. Tie the halves back together with kitchen string, as you would a joint of meat. The squash will shrink a little as it cooks, so make sure you make the knots nice and tight. Rub the outside with a little more oil and season well. Alternatively, wrap well in foil.
Place in a roasting tin lined with some foil or parchment and cook for 1 hr. Check if the squash is cooked by inserting a knife, it should go in with little resistance and the flesh should feel soft. Scatter any remaining stuffing around the squashetta and return to the oven for another 5 mins to warm through. Mix the tahini, lemon juice, honey and a drizzle of water to make a dressing. Carve the squash into slices and serve drizzled with the tahini sauce and scattered with parsley.