Preheat the oven to 220C/200C fan/Gas 7. Roughly dice the butternut into cubes, place on a baking tray, drizzle in olive oil, salt, dried rosemary and thyme, toss to coat, then pop in the oven for 20 minutes.
Meanwhile, chop the leeks and mushrooms, chop two of the sage leaves then add all to a frying pan with a drizzle of olive oil, season heavily with salt and pepper and fry for 5 minutes. Once the butternut has cooked, add to the pan with the leeks then add the vegan cream, tossing to ensure everything is coated. Turn off the heat and leave to cool.
Remove the pastry from the fridge and roll out onto a flat surface. Fold the sheet of pastry in half, then cut a circle shape (this will form the base and the dome on top) place one of the circles on a lined baking tray, then spoon on the filling leaving a 2cm gap around the very edge so you can seal the pie. Once you've added the filling to one half, add the other circle of pastry on top and pinch the edges together, you can create a pattern or simply overlap and pinch with a fork.
Brush the melted butter and tamari mix over the pie crust, score with a pattern then place in the fridge to cool for around 5 minutes (enough time to preheat the oven to 220C/200C fan/Gas 7). Pop in the oven on the middle tray for around 25-30 minutes or until the pastry has turned golden and flakey. Enjoy with gravy or with steamed vegetables.