Preheat the oven to 200°C / 400°F and line a baking sheet with parchment paper.
Remove the pastry from the freezer and let it sit for a few minutes if it is too firm to roll.
On a lightly floured surface, roll the dough into a roughly 30 cm (12-inch) circle, about 3–4 mm thick.
Transfer the pastry to the prepared baking sheet.
Spread the frangipane over the pastry, leaving a 4-5 cm (2-2.5 inch) border around the edges.
Arrange the sliced plums over the frangipane, slightly overlapping them. Drizzle over any excess liquid from the bowl onto the plums.
Fold the pastry border up and over the edges of the filling. You can create loose pleats as you go around the galette. Make sure the pastry tightly covers the plums.
Brush the pastry with beaten egg and sprinkle with granulated or coarse sugar. Sprinkle with the flaked almonds, if using.
Bake until the pastry is deeply golden and crisp, and the frangipane is puffed and set. The plums should have sunken slightly into the frangipane as well (about 40-45 minutes). Keep an eye on the tart for doneness/browning after about 25 min.
If the pastry edges are browning too quickly, loosely cover them with foil during the final part of baking.
Allow the galette to cool before slicing.
Serve warm or at room temperature. My favourite way to enjoy this is with vanilla ice cream.
This galette lasts up to 5 days and should be refrigerated and covered properly. Reheat leftovers in the microwave and serve warm.