First, use a veg peeler to peel the butternut squash and heat oven to 200C/180C fan/gas 6. Then using, a large, sharp knife cut a few 1cm thick ring-slices from the bulbous end, and a few small solid slices from the top end. Set aside the slices, and dice the rest into 1-2cm chunks. Toss the chunks with 1 tbsp of the sunflower oil on a baking parchment-lined tray and roast for 20-30 mins until tender and golden.
Meanwhile prepare the tin. It’s worth taking some time to do this, as it’ll be the top of your cake in the end – and the bit you want to look impressive! Line the base of a deep, round, 25cm tin with a sheet of baking parchment. Brush the new base and sides with some oil, then start to arrange the rings in the base. You want to get in as many as you can, overlapping a bit like the Olympic rings. Snuggle in as many flat as you can, then sit your overlapping ones on top, cutting out bits of the squash, so they’ll also sit flat too. Any leftover trimmings, put in a microwave-proof bowl with a splash of water, cover with cling film and microwave on High at 2 min intervals until tender – around 5-6 minutes.
In a separate pan, soften the onion in the last 2 tbsp of sunflower oil, over a very low heat so it doesn’t brown. Stir in the chopped sage, half the rosemary, and the garlic and mace, and cook for another few mins until fragrant. Mix the ground chia seeds or linseeds with 4 tbsp of water, and set aside with the cooling onions, until gluey.
Meanwhile, roughly chop half the chestnuts but keep them chunky. Put the other half in a food processor with half the lentils, the microwaved squash and one-third of the roasted squash. Pulse to a mashed mixture. Tip this into a large mixing bowl with the softened onions, breadcrumbs, ground seed mixture and 1 tsp salt. Mash everything together really well to thoroughly mix, then more gently stir in the chopped chestnuts, followed by the whole lentils, and finally the remaining roasted squash chunks. Carefully press this mixture around and over the squash rings in the prepared tin. Level off the top, making sure it is tightly packed, then cover with foil. The cake can now be chilled for up to 24 hours before continuing.
To bake, heat the oven to 180C/160C fan/gas 4. Put the cake (still foil covered), into the oven on a middle shelf and bake for 1 hr.
To serve, heat the rapeseed oil in a small frying pan and sizzle the reserved sage leaves with the remaining rosemary sprigs for a minute. Loosen around the sides of the cake with a round-bladed knife, then sit a serving plate inverted on top, and carefully flip the plate to turn out the cake. Spoon over the sizzled sage and rosemary leaves and herby oil, and serve at the table – for cutting into wedges in front of everyone.