Cook green lentils following instructions on a packet. Once cooked, drain and leave to cool down.
In the meantime, pour hot water over dried porcini mushrooms, just enough to submerge and leave for 15-20 minutes for the mushrooms to rehydrate. Then chop them finely.
Heat a large non-stick frying pan over low heat. Melt the butter in the pan, then add the onions and sautee for 6-7 minutes until softened. Add the chopped chestnut and porcini mushrooms, a pinch of salt and cook for a further 7-8 minutes until the water has released and evaporated.
Preheat the oven to 200°C Fan.
Mash the cooled lentils into a rough paste (if you have a food processor, just blitz them for a few seconds). Add the cheddar, cranberries, pistachios, breadcrumbs, thyme, soy sauce, 2 tbsp of water, onion & mushroom mixture to a large bowl and mix. It may look a little dry but that's ok.
Line a small loaf tin with baking parchment and grease lightly with vegetable oil.
Add the roast mixture to the tin, pressing down into the corners and smoothing out the surface.
Place the roast in the oven for 40-45 min. The top should have browned and the roast should be firm when pressed. Leave the roast to cool in the tin for 5 minutes.
Gently turn the roast out onto a chopping board, let it cool slightly and scut into thick slices.
Serve the roast with lots of gravy and all your favourite vegetables.