Preheat the oven to 200°C. Mix the butternut squash with the oil and za'atar in a large bowl. Spread out onto a baking sheet in a single layer and place in the oven. Cook for 45 minutes.
While the squash is cooking make the polenta. Bring the stock and milk to the boil in a large saucepan. Pour in the polenta and whisk until smooth. Cook for 10 minutes then add the butter and goat's cheese. Stir through and keep warm until ready to serve.
Make the herby tahini sauce by putting all of the ingredients into a food processor and blend until smooth. Serve the polenta topped with a serving of squash and a drizzle of tahini sauce.