Separate the egg white and yolk. Add 25ml cold water to the yolk and whisk until smooth.
Put the flour, butter, sugar and salt in the food processor. Or in a bowl if making by hand.
Pulse the food processor - or, if doing this by hand, rub together lightly between your fingers - until you have an even, slightly yellow crumb that resembles breadcrumbs or ground almonds.
Pulse again very briefly - or mix with a cutlery knife - until the pastry starts to clump.
Remove the pastry from the food processor / bowl and gather into a ball.
Place on clingfilm and cover with another piece, then press out with your hand into a disc about 1-1.5cm thick (½ inch). Wrap and place in the fridge to chill until firm but flexible.
Line the pastry case
When chilled to very firm, unwrap and roll out to about 3mm (just under1/8 Inch) thick - the disc will about 30cm (12 inches) wide.
Use the pastry to line your tart case, carefully lifting and pushing the pastry so that it sits flush in the corners with no gaps for air.
Use your rolling pin to trim away the excess pastry, then place the tin back into the fridge until hard.
Blind bake the pastry case
Preheat the oven to 200C (180C fan).
Scrunch up a large piece of greaseproof paper (baking parchment) and use it to line the pastry case, then fill with baking beans, ensuring that the sides are supported all the way round.
Place the tart on a baking sheet and place in the oven to bake for 20 minutes. Remove from the oven, spoon out the baking beans and remove the paper.
Return the tart to the oven for a further 10-15 minutes until the base is sandy with no gray patches.
While the pastry is still hot, if you like you can brush the base and sides (but not the top) with the reserved egg white. This will ensure it doesn’t become soggy.
Make the frangipane
Put the sugar and softened butter in a bowl. Beat until pale and fluffy.
Add the beaten eggs a little at a time, whisking after each addition.
Sift in the ground almonds and flour and fold through.
Fill and bake your mincemeat frangipane tart
Spoon the mincemeat into the case and level off.
Spoon the frangipane batter on top and level off.
Scatter the flaked almonds on top.
Return the tart to the oven and bake for 30-40 minutes until gently golden all the way across. Keep a close eye on it in the final 10 minutes and cover the edges with a collar of foil if you’re worried the edges are browning too much.
Allow to firm up in the tin for 10 minutes, and then gently release from the tin and transfer to a wire rack to cool completely.
Ice the tart
To ice your tart, sift the icing sugar into a bowl and add the lemon juice. Mix together until you have a smooth, thick icing.
Drizzle or pipe the icing all over the top of the tart.
Add some silver sprinkles, if desired, then allow to set.
When you’re ready to serve, slice with a large, sharp knife and wipe between slices for a neat finish.