Mix the flour, salt, sugar and yeast in a bowl. Add the water, egg and butter, knead for approx. 10 mins. to form a soft, smooth dough. Cover and leave to rise at room temperature for approx. 1½ hrs. until doubled in size.
Combine the almond mixture with the lemon juice.
Divide the dough into thirds. On a lightly floured surface, roll each piece of dough into a rectangle (approx. 15 x 30 cm). Spread one third of the almond filling on top of each piece, roll up from the long edge and shape into rolls approx. 40 cm long. Braid the three strands of dough to form a loaf, place on a baking tray lined with baking paper. Cover and leave to rise for a further 30 mins. Brush with egg, sprinkle with flaked almonds and sugar crystals.
Approx. 40 mins. in the lower half of an oven preheated to 180°C. Remove from the oven, slide the bread onto a rack and leave to cool.