Pre heat the oven to 200°C / 180°C fan / gas 6.
Place the butternut squash, thyme and rosemary on a baking tray. Drizzle over 2 tbsp of olive oil, season generally with salt and pepper and stir through to ensure the ingredients are well combined. Place in the oven to roast for 30 minutes.
In a large saucepan add 1 tbsp of olive oil and 1 tbsp of margarine. Warm over a low heat and add the red onion. Sauté for 10 minutes until softened and starting to caramelise.
Add the garlic and fry until lightly browned and fragrant, approximately 1 minute. Next add the mushrooms, balsamic vinegar, soy sauce and chilli flakes and cook for 5 minutes.
Turn off the heat and stir in the chopped chestnuts, dried cranberries and roasted butternut squash. Taste and season generously with salt.
Increase the oven temperature to 220°C / 200°C fan / gas 7.
Remove the pastry from the fridge. Cut the block in half and roll out both blocks to form a rectangle shape approximately the thickness of a pound coin. Place the pastry sheet on a baking tray.
Spoon the butternut filling onto the sheet, leaving a 1 inch border around the filling. Brush some of the oat milk over the exposed edge and then carefully place the second pastry sheet on top.
Crimp the edges together and trim off any excess. Score several slashes on the lid of the pie. Decorate the top with the excess pastry and finish by brushing the pie lid with oat milk and a sprinkling of the seeds.
Place the pie in the oven and roast for 30 minutes and the pastry is crisp and golden brown