The night before you want to bake, prepare the Leaven. Mix the starter with 100g flour and water and leave covered overnight.
Baking day
Add the rest of the flours and the water to the gloopy mix you made last night. Mix as best you can, cover and leave for 20 mins. A constant temp is ideal here, if you have a proving setting on your oven it will be about 40 degrees.
After the resting add the salt, don't freak out because it looks like a lot, it needs it! Let it rest for 30 mins.
Stretch and fold and leave to prove for 30 minutes. Do this 3 times.
Flour your work surface and gently form the bread into a ball. Use a fine flour like rice to dust a bowl for the bread to rise in and dip the smooth side of the bread into rye flour before placing in the bowl.Cover gently and let rise for 1 to one and a half hours.
Heat up a large heavy lidded pot in the oven - 250 degrees if you can.
Tip the risen loaf into the pot and slash a pattern into the top.
Cook with the lid on for 20 mins and the lid off for 15. The loaf should sound hollow when tapped on the bottom.