For the levain
50g strong wholemeal flour
50g strong white flour
100ml warm water (approx 30degrees C)
1tbsp sourdough starter (you might need to obtain this from your local sourdough baker if you don't already make sourdough)
Make the levain the night before (or in the morning if you want to prepare the dough in the late afternoon) - mix all the ingredients and then leave covered in a warm place (ideally about 20-25degrees C) - it will expand so put it in a container with some extra space in it.
When you are ready to make your dough you will need 130g of this levain - put the rest in the fridge and this will be your starter for next time.
600g strong white flour
70g stong wholemeal flour
470ml warm water (approx 30degrees C)
Put all the ingredients in a large bowl and combine til no traces of dry flour are left
Leave to stand, covered (eg with a clean teatowel/showercap) for 20-30mins
Then add 15g salt and 30ml warm water and mix again til combined. When mixed form the dough into a round by doing a 'fold' - gently pick up the side of the dough from the bottom (to fit your hand) and fold into the middle - turn the dough a quarter turn and repeat - repeat 8 times so you have been round twice and then turn the dough over so the folded side is at the bottom. Cover again
Leave for 30mins
Fold (as described above) - this is fold 2
Repeat for a total of 6 folds with half hour rest in between each one
Then turn the dough out onto a lightly floured surface, lightly flour the top of the dough and your hands, then keep turning the dough and gently tucking the sides underneath until the top is lightly stretched smooth and the dough is in a fairly compact sphere.
Leave for 20-30mins
Shape your dough to the size and shape of your banneton/s by folding over several times to knock some of the air out. This amount of dough will make either 2 x 650g loaves or 1x 1.3kg loaf - if making 2 loaves cut the dough in half before final shaping.
Flour your bannetons and put in the dough, smooth side down so when you turn it out this will be the top, cover and put in the fridge. The best way to cover is to put it in a plastic carrier bag which you can trap some air in, so the top of the bag doesn't stick to the top of the dough.
Leave for 6-16hours in the fridge (overnight if you started in the morning).
Preheat oven as hot as it will go, if you have a pizza stone put this in to preheat also - otherwise a baking tray will do. Put a small roasting tray/similar to preheat on the bottom shelf too.
Boil the kettle
When the oven is hot take another baking tray, big enough to fit your loaf on, turn upside down so there are no edges and cover with a silicone sheet/something slidey and non stick. Turn your loaf out of the banneton onto this sheet. Make some slashes in the top with a sharp knife to allow the loaf to expand. Open the oven and slide the loaf on its silicone sheet onto your preheated tray/stone.
Quickly pour a couple of centimetres of boiling water into your roasting tray to make steam and shut the oven door. Turn the oven down to 220 degrees C
After 15mins briefly open the oven door to let the steam out
After another 15mins the loaf/s will be cooked - cooking time is the same whether 1 big loaf or 2 smaller ones. You can give it 5mins longer if you prefer a darker crust.