Step 1

Begin by making the stuffing. Preheat an oven to 180°C/gas mark Wash and chop the leek into 3cm pieces and place in a roasting tray. Add the mushrooms and garlic, then drizzle with oil and season. Cover with foil and roast until soft (about 30 minutes)

Step 2

Meanwhile, add the mayonnaise to a bowl and finely grate in the garlic. Season with salt and pepper and stir to combine

Step 3

Once the vegetables are soft, transfer the mixture to a blender or food processor and blitz with the chestnuts, bread and sage. The moisture from the roasted vegetables should bind the mixture but don't over-blitz the mixture – you want some texture. Season with salt and pepper

Step 4

Line a loaf tin or baking dish with baking paper then spoon in the stuffing mixture. Bake for another 30 minutes

Step 5

Place a small frying pan over a high heat and add a drizzle of oil. Add the sliced mushrooms with a pinch of salt and cook for a few minutes until golden

Step 6

Now it's time to assemble the sandwich. Slice the bun in half and spread cranberry sauce over the bottom. Top with a few thick slices of the stuffing

Step 7

Top with a layer of mushrooms and a handful of spinach. Spread the aioli generously over the lid of the sandwich and place on top