Boil the potatoes in lightly salted water until soft (or use left-over potatoes). Drain and set aside.
Preheat the oven to 180 °C.
Fry the onion in 2 Tbsp olive oil in a thick-bottomed pan on medium heat until soft and translucent but not browned, 10-12 minutes. Add the potatoes and mash with a fork. Transfer to a bowl and add 50 ml/⅕ cup milk, 1 Tbsp extra virgin olive oil, flat-leaf parsley and pul biber (if using). Stir until combined and season.
Whisk the 200 ml/⅘ cup milk and 50 ml/⅕ cup olive oil together in a bowl.
Make plenty of space on your kitchen counter and put baking parchment on two baking sheets (you may need just one, depending on how many böreks you end up with). Open the packet of filo pastry and cover with a damp cloth to avoid the pastry from drying out (it can dry out pretty quickly).
Take two sheets of filo pastry and put on the kitchen counter (I usually do this on a baking parchments to avoid the filo sticking to the counter). Brush the whole of the top side of the pastry with the milk/oil mixture (c. 1 ½-2 tbsp). Add about 2 tbsp of the filling in a thin strip 1-2 cm (½-⅔ in)from the top. Leave the same distance to the left and a little more to the right (3-4 cm/1 ⅓ in). If you’ve not made it before, you’ll need less filling than you think. Roll up from the end where you put the filling until you have a long sausage; brush a little extra milk/oil mixture at the end to seal if needed. Swirl the börek sausage into a rose/swirl shape, making sure to keep the end with 3-4 cm/1 ⅓ in clearance out (i.e. the left hand side should be in the middle of the rose, the right hand side should finish on the outside). Flatten the end with your fingers and put underneath the börek rose to seal. Set aside on the baking parchment.
Repeat until you’ve got no filling left. If you run out of the milk/oil mixture, just make a small amount extra, using a ratio of 4 parts milk 1 part olive oil.
Brush the rose böreks first with the milk/oil mixture until the top and sides are moist, then with the egg yolk. Sprinkle nigella seeds or sesame seeds on top.
Bake the börek until golden and cooked through, 30-40 minutes. If you need to use both trays, swap places after 20-25 minutes to ensure they are evenly browned. If you managed to fit them all onto the same tray, just put it in the middle of the oven.
Leave the böreks to rest for at least 10 minutes before serving warm (but not hot) or at room temperature.
Tips & notes
If you love cheese, add 75 g/2.5 oz crumbled feta cheese to the filling.
The filling can be prepared the day before and stored in the fridge.
Börek keeps well for 2-3 days stored in an air-tight container in the fridge. Reheat in a non-stick pan on medium/low heat to regain crispyness.
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