Combine the flaxseeds with 4 tbsp water in a small bowl, and leave to thicken. Heat the oven to 200C/180C fan/gas 6. Toss the squash in a roasting tin with 1 tbsp olive oil, the za’atar, cumin and coriander. Cover the tin with foil and roast for 30-35 mins, or until the squash is tender but still holding its shape. Oil a 900g loaf tin and line with baking parchment.
Heat the remaining oil in a frying pan, and fry the onion, celery and mushrooms for 10-15 mins, or until softened. Stir in the garlic and miso paste.
Tip the pistachios, chestnuts, breadcrumbs and parsley into a bowl, then stir in the flaxseed mixture, fried veg and roasted squash. Season well.
Put the cranberries in a pan with the sugar and cook for 3-4 mins, or until the sugar has dissolved and the cranberries are just starting to soften. Spoon into the base of the loaf tin, then leave to stand for 5 mins to set slightly. Top with the nut roast mixture, patting it down firmly and smoothing the top with the back of a spoon. Cover with foil and bake for 25 mins, then uncover and bake for 25 mins more. Leave to rest for 15 mins, then invert onto a serving plate, removing the parchment. Top with extra chopped pistachios and pomegranate seeds, then serve.