Preheat the oven to 355°F (180°C) with the fan setting on.
To make the almond cream, mix all the cream ingredients by hand using a whisk until a thick and smooth cream forms. Chill in the refrigerator (the cream can be stored for up to two days).
Roll out the puff pastry on a baking sheet lined with parchment paper. Spread the almond cream evenly in the center of the pastry. Sprinkle fresh blueberries on top. Cut strips along the edges of the dough and braid them over the filling.
Brush the pastry with the egg, sprinkle with sliced almonds, and bake for 30-40 minutes, or until the pastry is golden brown.
While the pastry is baking, prepare the sugar syrup by dissolving the sugar and water in a small saucepan over low heat.
After baking, brush the pastry with the sugar syrup, let it cool slightly, dust with powdered sugar, and slice. Store the pastry at room temperature in an airtight container for up to two days.