Add 50g sourdough starter, 50g bread flour and 50g water to a see-through container with straight sides. Mix well.
Leave to ferment, covered in a warm place. Once it has grown to the double the size you need to mix the dough.
Make the blueberry jam - morning
Put 250g blueberries in a small pot. Add 125g sugar, 3 tbsps cornstarch and 50g water.
Mix to combine everything. Bring the mixture to a boil.
Let simmer for about 10 minutes. Turn off the heat and let the mixture sit in the pan for about 15 minutes to a cool a bit.
Add the mixture to a bowl and cover with aluminium foil. Let cool in the refrigerator until needed.
Mix the dough - around noon
Add 450g bread flour, 150g levain, 4 eggs, 30g sugar, 15g salt and 60g milk to a bowl.
Put your index and long finger in the middle of the dough and start moving it in circles, for each revolution make the circle a little bigger so you get more and more flour into the fluids,.
Once that gets difficult, grab the back of the dough with your hand, stretch it up and in over the dough, turn the bowl a bit and repeat until all the flour is completely hydrated.
Leave the dough to rest for 30 minutes under a damp dish towel.
Bulk fermentation - around 12:30/12.30 p.m
Perform four sets of stretch and folds spaced out by 30 minutes.
After the fourth set of stretch and folds, perform a windowpane test to check the gluten development. If it doesn't pass perform one more stretch and fold and let the dough rest for 30 minutes and test again.
Once the windowpane passes, take the butter out of the refrigerator and cube it. Rest the dough for 30 minutes.
Now it's time to add the butter. Put a small handful of cubes on the dough and start to push it and massage it into the dough. Keep going until you cannot feel any lumps anymore. Then add another handful. Repeat until you have added all the butter.
Then let the dough ferment until it has expanded by 30-50% in volume.
Put the dough in the refrigerator for a minimum of 2 hours, a maximum of 48.
Shape and final proof
Flour your work surface liberally. Dump the dough out and tease it into a rectangle.
Roll it out so that it's about the width of your pan minus 1.5cm/½". Roll it pretty thinly so that the bread has to rolled several times to be one round loaf.
Apply the blueberry jam on the dough. At the end you will start rolling you can go to the edge, but at the other end you should leave about 3cm/1" room without any jam. On the sides you should leave about 1.5cm/½" room without jam.
Then roll up the dough pretty tightly.
After you have a roll of dough, fold the ends underneath the bottom to seal the jam inside.
Grease your tin liberally with butter or baking spray and place the dough in the tin.
Then cover the tin with a damp dish towel and let it proof until it has doubled in size.
Bake the bread
Pre-heat the oven to 200ºC/390ºF/Gas mark 6.
Beat 1 egg with 1 tbsp milk.
Brush the bread lightly with the egg wash.
Bake for about 60 minutes or until the inner temperature reaches 98°C/208ºF.
If the bread starts to get too brown during baking cover it with a piece of aluminum foil.
Once the bread is out of the oven, let it cool for 10 minutes and then take it out of the tin and let cool completely.