Preheat the oven to 120℃/Fan 100℃/Gas ½. Place a loaf tin in a deep baking tray or roasting tin and preheat in the oven.
Melt the butter in a small frying pan over a medium heat.
Place 90g/3¼oz of the parfait reduction, reserving the remaining reduction to serve with the finished parfait, in a food processor or blender. Add the mushroom purée and eggs and blend.
Add the melted butter and continue blending until fully combined. Season with salt to taste.
Remove the loaf tin from the oven and pour in the terrine mixture. Cover with a double layer of kitchen foil and place back on the baking tray.
Place the tray in the oven. With the oven door open, carefully add boiling water into the baking tray up to the level of the parfait in the loaf tin.
Cook for 20 minutes. The parfait should be just set and have a slight wobble in the centre.
Remove the tray from the oven very carefully. Cool the parfait to room temperature for 30 minutes and then chill the loaf tin in the fridge overnight to set.
Remove the parfait from the fridge and, using a spatula, lightly scrape off and discard any thick skin that may have formed. Place the parfait in an airtight container if storing further.
To serve, scoop a generous amount of parfait onto a plate and garnish with the remaining parfait reduction and freshly shaved mushrooms. Serve with the grilled sourdough.