Preheat the oven to 200°C, fan 180°C, gas 6. Squash skin should be pierced all over with a knife, then put to a baking tray and roast for 45 minutes to an hour, until cooked. Allow to cool for 10-15 minutes.
Meanwhile, bring the stock to a boil in a saucepan. Cover and heat for 10 minutes, or until the cauliflower is soft, adding the garlic cloves and bay leaf (if using).
Transfer the cauliflower and garlic to a blender with a slotted spoon, then add 120ml of the cooking liquid.
Blend until smooth, adding more cooking liquid as needed to achieve the desired consistency. Combine the nutritional yeast, soy sauce, and truffle oil in a mixing bowl. To bring the sauce together, blend it once more. Set aside.
With a sharp knife, remove the top and tail of the squash, then stand on one end and cut away the skin. In a frying pan, heat 2 tbsp oil over medium heat until hot.
Carefully add the squash and cook for 20 minutes, turning occasionally, until lightly browned all over, adding 1 tbsp oil if necessary.
Combine the pepper, paprika, sugar, and 14 teaspoon salt in a mixing bowl. To coat the squash completely, press all over it.
To allow the seasoning to set, transfer to a baking tray and bake for another 5 minutes.
In a small saucepan, reheat the cauliflower sauce over low heat. Set aside the seeded section of the squash while slicing it into rounds.
Serve with the cauliflower sauce, chopped parsley, and pistachios on top.