To make the pastry, sieve 500g of plain flour and 100g of icing sugar into a large bowl. Chop 250g of unsalted butter into cubes, then, using your fingertips, gently work it into the flour and sugar until the mixture resembles breadcrumbs (or simply pulse in a food processor). Beat 2 large eggs with 4 tablespoons of whole milk and 1 teaspoon of vanilla bean paste, then add to the flour mixture and gently work it together until it forms a ball of dough. Divide the dough in half, cover each piece and pop into the fridge to rest for 30 minutes. Roll one of the pieces of pastry out into a round, about ½cm thick. Lightly grease a 24cm non-stick loose-bottomed tart tin with vegetable oil. Line the base and sides with the pastry. Prick the bottom with a fork, then place back in the fridge until needed. Preheat the oven to 180°C/350°F/gas 4. Meanwhile, make the filling. Trim 1kg of rhubarb and peel and core 500g of eating apples, then chop into 3cm chunks and place into a large pan with 125g of golden caster sugar, and a splash of water. Simmer on a medium heat for 15 minutes, or until tender, then strain through a colander, catching the liquid in a bowl. Tip the strained liquid back into the pan and simmer over a medium heat for 5 minutes, or until thick and syrupy. Remove from the heat and leave to cool. Scrunch up a large piece of greaseproof paper, then unwrap it and use it to line your pastry case, pushing it right into the sides. Fill the case with rice or baking beans, and bake blind for 10 minutes. Carefully remove the rice or baking beads (save for another bake) and return to the oven for a further 10 minutes, or until golden and crisp. Pour 500ml of whole milk into a pan over a medium-low heat, whisk in 3 level tablespoons of custard powder, the remaining 35g of golden caster sugar, 2 teaspoons of vanilla bean paste. Whisk in a figure of 8 until the mixture becomes really thick, then pour into the pastry case and leave to cool. For your pie lid, roll the remaining pastry out into a circle, just under 1cm thick (it will be larger than you need, but the excess can be used for decorating). Carefully spoon the apples and rhubarb over the custard base. Beat the remaining egg, then use it to eggwash the edges of the pie. Top with the pie lid, gently tuck in slightly, then roll over the top with your rolling pin to trim away the excess pastry. Use the excess pastry to decorate the top of your pie, however you like. Brush with a little more beaten egg to glaze, then toss 50g of flaked almonds in 1 teaspoon of icing sugar and a tiny splash of water, then sprinkle over the top. Bake for 40 to 45 minutes, or until golden. Delicious served with more custard and the beautiful fruit syrup drizzled over.