For the nut roast crust, roughly chop the pistachios, then spread out on a baking tray with the seeds, then toast in the oven for 5 to 6 minutes.
When the time's up, transfer to a food processor, along with the chestnut purée, suet and maple syrup, and blitz until the mixture comes together into a ball. It will be very sticky to begin with, so stop and scrape the sides as you go.
Place a large sheet of baking parchment on a work surface and sit the dough on it. Then, with lots of gluten-free flour on your hands and rolling pin, roll out the dough as thinly as possible (less than 5mm).
If you’re making individual tartlets, oil and flour four 10cm loose-bottomed tart tins, then cut out the pastry to size. Or, roll out the dough and cut to the size of a large baking tin, then transfer to the tin using a fish slice.
Prick the dough all over with a fork, cover with baking parchment, fill with baking beans or uncooked rice and bake blind for 12 to 15 minutes. Leave to cool completely in the baking tray, as it will be quite delicate straight from the oven. Keep the oven on.
For the topping, chop the sweet potato into cubes, then place on a baking tray. Toss with a little oil, then bake for 25 minutes, or until soft.
Melt the butter in a frying pan over a medium heat, slice and add the mushrooms along with 1 teaspoon of cracked black pepper, and sauté for 6 to 8 minutes, until the mushrooms brown.
Blitz the roasted sweet potato in a food processor with the crème fraîche, nutmeg and ½ a tablespoon of cracked black pepper (or to taste) to a smooth, creamy consistency.
To assemble the tart, crumble the blue cheese over the base, arrange the sautéed mushrooms on top, then finish with the sweet potato mix.
Sprinkle with linseed, roughly chop and scatter over the pistachios followed by the sunflower seeds, then add a drizzle of rapeseed oil, and pop back in the oven for 6 to 7 minutes until it just starts to brown. Serve hot with gravy and roast vegetables.