Tip the remaining 500g of strong white flour into the bowl of a food mixer, along with the remaining yeast, salt and sugar. Using a dough hook, combine then add the starter mixture.
Add 100g of softened butter, and 150ml of tepid water. Mix on low speed (1 or 2) for 20 minutes. You'll end up with a firm dough.
Tip the dough on a clean work surface. Take the left side and bring it into the middle, then do the same with the right - like an envelope.Repeat with the bottom and top. Grease a large bowl and put the dough into it, fold side down. Cover with a tea towel and leave for one hour.
Whack the butter with a rolling pin to flatten it. Then roll out 330g cold butter between 2 pieces of parchment or silpats into a 17 x 19 cm rectangle. Put this in the fridge.
Make space in your freezer for a small baking tin (30 x 21cm). Line the baking tin with parchment paper.
Lightly flour a work surface and tip the dough on to it. Pat it out to a rectangle of 25cm x 19cm. Put this into the lined tin. Cover with clingfilm and put in the freezer for 20 minutes.
Tip the dough onto the lightly floured work surface and dust the top with flour. roll out to a 40 x 20cm rectangle, about 1.5cm thick. Put the chilled butter into the middle and fold the two ends of the dough over the top to cover the butter. Press together to seal.
Roll out the butter filled dough to 55cm x 22cm. Try to maintain a rectangle with straight sides. Tuck any escaping butter back in. Then fold it in thirds, like a book. Put it back into the tin, cover and freeze for 20 minutes.
Repeat the above step twice more. Put something good on Netflix for while you wait between steps. I watched Glow, series 2. The wrestling motivated me to do the physical act of rolling.
Dust the top with flour, roll out to 60 x 22cm. Cut in half so you have two pieces 30 x 22cm. Cover and freeze for 20 minutes.
Line a large baking sheet with parchment or silpat. Lightly flour the work surface and put one rectangle of dough on it, putting the other in the fridge.
Roll it out to 45 x 22cm. Measure 10cm along the long straight edge, then cut up to the left hand corner of the dough to make a triangle. To make a second triangle, measure 10 cm along the top and cut down to the bottom. Repeat. You should be able to get 8 triangles.
One at a time, roll each triangle to 30cm long, concentrating more on the wide part of the triangle. Toll up the dough from the base to the top. You will have a typical croissant shape. Place on the baking sheet. Repeat with the other triangles.
Remove the dough in the fridge and repeat the last two steps.
You will now have 15 or 16 croissants on the baking tray. Brush with egg (starting from the inside and working outwards so that the layers of dough do not stick together and prevent the croissants from rising) and leave to prove for 2 hours or until doubled in size.
At this point you could freeze the pastries, ready to bake another day. Or bake half, freeze half.
Preheat the oven to 190ºc. Lightly brush the croissants with egg again. Bake for 20 minutes, turning the baking tray around halfway through. Cool on a wire rack.