The blood-orange season begins in December, but the fruit tends to be most abundant in January and February, when the dark, almost purple moro variety ripens for just a short few months. Blood oranges are generally smaller (and more juicy) than regular oranges, but if yours are larger, reduce the quantity to six.
Make the pastry. Tip the flour into a large mixing bowl and add the sugar, cinnamon (or other ground spice) and salt. Using your fingertips, rub the butter into the flour until the pieces of butter are the size of peas. Drizzle in the ice-cold water and, using your hands, gather the mixture into a scrappy dough without over-mixing. Add a little more water if needed, but maintain the large butter pieces throughout the dough. Flatten the pastry into a disc, wrap it and chill it for 1 hour, until firm.
Roll out the pastry on a lightly floured work surface to a 30cm disc. Trim the edges to neaten. Slide the pastry onto the lined baking sheet and chill it for 30 minutes while you prepare the filling. Meanwhile, heat the oven to 190°C/170°C fan/375°F/Gas 5.
Using a wooden spoon, beat together the butter and sugar until pale and creamy. Add the ground almonds, egg yolk, orange zest and salt, then mix again until combined.
Using a small, serrated knife, cut a thin slice off the top and bottom of each blood orange. Stand the oranges upright and, working around each fruit from top to bottom, cut off the skin and pith in strips, so that you have beautifully neat, peeled fruit. Cut each orange into 5–10mm-thick slices.
Spread the almond mixture over the pastry disc, leaving a 4cm border around the edge. Arrange the orange slices on top of the almond paste. Fold and roll the pastry edge over to form a crust.
Lightly beat the egg white and brush this over the pastry crust. Sprinkle caster sugar over the pastry and oranges and scatter the galette with the flaked almonds. Bake for 35–40 minutes, until the pastry is crisp and golden and the oranges start to caramelise at the edges. Serve warm or at room temperature with crème fraîche.