In a large bowl, whisk together the all-purpose flour, almond flour, and salt. Add the cold, cubed butter to the flour mixture. Use a pastry blender or your fingers to cut the butter into the flour until the mixture resembles coarse crumbs. Slowly add the ice water, one tablespoon at a time, mixing until the dough just comes together. Be careful not to overmix.

Form the dough into a disc, wrap it in plastic wrap, and refrigerate for at least 30 minutes.

While the dough is chilling, prepare the plum filling. In a large bowl, combine the sliced plums, granulated sugar, all-purpose flour, and ground cinnamon. Toss gently to coat the plums evenly.

Preheat your oven to 375°F (190°C). On a lightly floured surface, roll out the chilled dough into a 12-inch circle. Transfer the dough to a baking sheet lined with parchment paper. Sprinkle the almond meal evenly over the center of the dough, leaving a 2-inch border.

Arrange the plum mixture over the almond meal, leaving the 2-inch border clear. Dot the plums with small pieces of butter.

Gently fold the edges of the dough over the plum filling, pleating as you go. The center of the galette should remain open.

Brush the folded edges of the crust with the beaten egg and sprinkle with the sliced almonds.

Bake the rustic plum galette in the preheated oven for 35-40 minutes, or until the crust is golden brown and the plums are tender and juicy. Keep an eye on it; if the crust starts to brown too quickly, you can tent it with foil.

Remove the galette from the oven and let it cool on the baking sheet for at least 15 minutes before slicing and serving.