Rub together (or use a food processor to pulse) the flour, butter, icing sugar and salt into a breadcrumb-like consistency. Add the vinegar and cold milk, and press into a ball of dough. Divide the pastry into two not-quite-equal pieces, push into rectangles about 3cm thick, then wrap both and refrigerate for at least an hour, ideally longer.
The pastry is very buttery and can be tricky to handle, so roll out between two sheets of baking paper: the smaller one so that it’s the same size as your tin (this will be the lid); the other, big enough to line the base and sides; and both to 2-3mm thick. You’ll be able to break off bits that are not in the right shape and place them where they should be as you go. Refrigerate for at least an hour (again).
Combine the fruit in a bowl with the caster sugar and leave to macerate. After 20 minutes, add half the ground almonds, stir and set to one side.
Butter the baking tin, dust with flour, then line the tin with the larger pastry sheet. Use a knife to trim the pastry so it’s flush with the top of the tin, using the excess to patch up any holes or thinner areas.
Sprinkle the base with the remaining ground almonds then tip the filling in, ensuring an even distribution. Brush the edge of the pastry base with milk, then place the lid on top, pressing down firmly to seal the pastry together. Trim any overhang. Brush with milk, then add a liberal sprinkling of demerara sugar. Refrigerate one final time for at least 30 minutes (the pastry needs to be cold and the oven fully to temperature). Heat the oven to 200°C/180°C fan. Place the baking tin on a larger sheet (to catch any spilled juices) and bake for 45 minutes, until the pastry is hard and golden, with some of the fruit bubbling through. If after 35–40 minutes the pie is looking very bronzed, turn the oven down to 180°C/160°C fan but do keep it in for the full amount of time. Leave to cool for 10 minutes before serving with big dollops of crème fraîche.