Preheat the oven to 180°C/gas 4. Cut the liver into strips about 1cm thick. Toss in the flour and a little seasoning to lightly coat. Cut the bacon into strips the same size as the liver. Heat the oil in a heavy, deep frying pan, then quickly stir-fry the liver and bacon until lightly browned. Remove from the pan to a plate and set aside.
Add the shallots and carrots to the pan and cook, stirring frequently, until lightly coloured. Add the mushrooms and cook, stirring frequently, for a couple of minutes, then add the garlic, herbs, stock and sherry. Bring to the boil, stirring.
Return the liver and bacon to the pan and stir together. Transfer to a 1 litre pie dish, mounding the filling in the centre, and leave to cool while making the pastry.
Combine the flour, breadcrumbs, rosemary, suet and a little seasoning in a mixing bowl. Stir in enough cold water, about 5 tbsp, to make a soft but not sticky dough. Turn out onto a lightly floured work surface and gently roll out to an oval slightly larger than the pie dish.
Dampen the rim of the dish, then gently lift the pastry over the filling and press the pastry to the rim to seal. Trim off any excess pastry with a sharp knife, make a steam hole in the middle and press the back of a fork onto the pastry rim to decorate. Bake for 30–35 minutes until golden brown, then serve hot.
This recipe can be made ahead then cooled, covered and chilled for up to 48 hours. Reheat in a preheated 180°C/gas 4 oven for 20 minutes before serving. Make sure the filling is piping hot.