To make the pastry, sift the flour and a pinch of salt into a bowl. Add the butter and stir through to coat it with flour. Add enough of the iced water to make the dough come together. Gather into a ball and transfer to a floured work surface. Flour your hands and form the dough into a rectangular block. Roll this out to make a rectangle that is about 1 cm (½ inch) thick and has the short side closest to you. Fold up the bottom third of the rectangle and fold down the top third. Seal the edges lightly with a rolling pin. Turn the dough through 90 degrees and roll it out again into a rectangle that is about 5 mm (¼ inch) thick. Fold as before, turn the dough and roll out again. Do this once more, then put the dough in a plastic bag and leave in the fridge for 30 minutes. It can be frozen at this point if you won’t be using it immediately, or the pastry can be kept in the fridge for a couple of days.
Meanwhile, preheat your oven to 200°C (400°F/Gas 6). Melt the butter with the oil in a casserole dish on the stovetop. Brown the pork quite quickly, seasoning the browned sides with salt and pepper. Add the garlic and sage and cook for a few seconds before adding the brandy. Either ignite the brandy and let it burn out, or just let it evaporate. Add the mushrooms around the meat, seasoning them with a little salt and pepper.
Put the dish in the oven and cook for 20 minutes, turning the pork over once or twice and shuffling the mushrooms. The pork should be nicely browned and cooked through, but still slightly pink inside (it will get more time in the oven in the pastry jacket). Remove the pork to a plate to cool and leave the mushrooms with any juice in the dish.
Take your pastry out of the fridge and flatten it out on a lightly floured surface to make a rectangle that is about 30 × 25 cm (12 × 10 inches). Lay the pork fillet along one long side, towards the bottom, and roll up the pastry around it. Pinch the ends together and fold them up over the top of the fillet. Put on a baking tray lined with baking paper, reduce the oven to 180°C (350°F/Gas 4) and cook for about 30 minutes, or until the pastry is nicely golden and cooked through and the bottom is also golden and firm. Remove from the oven and leave to stand while you finish the sauce.
Put the casserole dish with the mushrooms on the stovetop and add about 125 ml (½ cup) water, scraping up any bits from the bottom. Bring to the boil, let it thicken slightly and then add the cream. As soon as it comes to a rolling simmer, remove from the heat and season. If the sauce seems too thin, leave it to simmer for a little longer.
Slice the pork and serve immediately with a little of the sauce spooned on the side. Don’t spoon sauce all over the pastry or it will become soggy.