1. The dough is made ahead as it needs to sit in the fridge for at least 3 hours and up to 3 days, I used mine after about 48 hours.
2. To make the dough, mix the flour, granulated sugar, yeast and salt in the bowl of a food processor or a standing mixer with a paddle attachment. Add the butter and whiz to combine, it will not get incorporated in the flour but still be visible in the shape of small peas.
3. Whisk together the egg, milk and 2 tbsp. water. Add the egg mixture to the flour mixture and fold it in with a spatula until absorbed – there’s no need to whiz it in the processor. Turn the dough out onto a piece of cling film, shape into a rectangle, wrap and chill for at least 3 hours, and up to 2 days.
4. When you’re ready for the folding stage, roll the dough out on a well-floured surface, using a floured rolling pin, to a rectangle of about 20 x 40cm. With the short side facing you, fold the dough in thirds like a letter, bringing the top third of the dough down, then folding the bottom third up. If it sticks, throw some more flour at it and use a dough scraper to detach it. Rotate the dough 90 degrees. Repeat the rolling and folding process, then rotate the dough once more and roll and fold again. As you work, dust the work surface, your hands and the rolling pin with flour as necessary. Wrap the dough in plastic wrap and refrigerate for at least 1 hour.
5. Repeat the entire three-times-rolling and folding process again – the dough should start to become smoother. If it’s still sticky, chill it for a bit longer. Wrap the dough and refrigerate for at least 3 hours, or overnight.
6. In a small bowl, mix together the sesame seeds, poppy seeds, almonds and pine nuts. Beat the egg white lightly with a fork.
7. On a floured surface, roll the dough out into rectangle 30 x 40cm. Brush the dough with egg white. Sprinkle the seed mixture evenly over the surface.
8. Line a large baking sheet with parchment paper. Starting from a short side, roll the dough up tightly. Using a bread knife or a cake wire trim off the ends, then slice the roll into 12 equal pieces. Arrange the pastries on the baking sheet and loosely cover with plastic wrap. Let stand until slightly puffed, about 1 hour to 1 hour 20 minutes. Heat the oven to 220C/425F/ gas 7.
9. In a small bowl, whisk the egg yolk with 1 tsp water.
10. Remove the plastic and gently brush the top and sides of the pastries with the yolk mixture. Sprinkle with salt and onion flakes. Bake for 20-25 minutes until risen and golden brown, lowering the temperature to 190C/375F/gas 5 after the first 15 minutes.
11. Cool slightly on a wire rack; serve warm or cold.