2) Preheat the oven to 180C/Gas 4. Place the plum wedges in a bowl and toss with the sugar and juice of half a lemon.
3) Remove the baking tray from the freezer. Working from the centre, arrange the plum wedges thickly over the dough, leaving about a 3cm border all around the edges. Fold the exposed edges of dough up and over the plums so that the galette looks something like a pizza with a border of crust all around the outside. Brush the dough with the beaten egg and sprinkle with more sugar.
4) Place on the bottom rack of the oven and bake until the crust is golden and the fruit is soft, about 1 hour. Remove from oven and allow to cool slightly on a baking rack. If using, sprinkle with the toasted almonds.
5) When the galette is no longer hot to the touch, slide onto a cake plate or pedestal. Serve warm or at room temperature.
For the pate sablee:
1) In the bowl of a food processor, combine the flour, sugar and salt. Pulse to combine. Add the butter and pulse the food processor on and off until the mixture resembles coarse sand with pea-sized lumps of butter still showing. Add the egg yolk and the ice water and pulse on and off until the mixture comes together but does not form a single mass.
2) Turn the dough out onto a clean surface and gather it up into a loose ball. Working with a small bit of dough at a time, smoosh the dough across the counter away from you with the heel of your hand and then scrape it up with a pastry cutter and gather it back together, forming a second ball. Repeat with the remaining dough until all the smooshed dough is in a single ball.
3) Flatten into a disk, wrap well in cling film, and leave to rest in the refrigerator for at least 30 minutes and up to overnight.