Preheat the oven to 180 degrees C (350F) and grease a loaf pan with olive oil.
Heat the olive oil in a large frying pan or skillet. Add the mushrooms, onion and bell pepper, and cook for 5 minutes, until they soften.
Meanwhile, add the pecans, almonds and pistachios to a blender or food processor, and blend quickly to form a crumbly mixture.
Transfer the cooked vegetables to a large mixing bowl together with the nuts. Add the sweet potato, chives, parsley, flax eggs, paprika, salt, cumin, garlic powder, gluten-free flour, tomato paste, mustard, tamari, red chilli pepper, and garlic. Mix together really well.
Transfer to the loaf tin and flatten down well using a spatula. Bake in the preheated oven for 40-45 minutes, until crispy and golden brown on top.
Allow to cool for around 5-10 minutes, then use a butter knife to loosen the nut roast around the edges. Place a serving board on top of the loaf pan and carefully flip upside down. Remove the loaf pan and serve.
To make the sweet potato purée, bake the sweet potato at 400 degrees C for 50 minutes, then peel it and mash using a fork or a potato masher.