Heat the oven to 180°C/160°C fan/gas 4. Melt the butter in a frying pan over a low heat. Add the onion, shallots, carrots and garlic, then fry gently for 5 minutes. Stir in the mushrooms and cook, stirring, for 5-10 minutes until softened.
Leave to cool for a few minutes, then transfer the mixture to a mixing bowl and stir in all the remaining ingredients, apart from the pomegranate seeds, until well combined. Spoon the mixture into the prepared tin and use the back of a spoon to pack it down and smooth the top.
Bake for 1 hour or until well browned and the nut roast feels firm to the touch. You may need to cover with a piece of foil or baking paper if it browns too quickly. Serve drizzled with the extra pomegranate molasses and scattered with the extra herbs and the pomegranate seeds, alongside the usual Christmas vegetables.