In a large bowl, dissolve the yeast and the sugar in lukewarm water.
Add half of the white flour and start mixing with a fork or wooden spoon till fully incorporated. Then add in the rest of the white flour and keep mixing well.
Add the salt and half of the wholegrain flower. Keep mixing with the spoon.
Add the rest of the wholegrain flower and keep mixing till you form a consistent dough. If necessary add more or less flour. Just make sure that the dough remains soft and hydrated.
Cover the bottom of the banneton (or bowl) with a generous amount of flour before putting the dough to rest inside. Sprinkle with some flour on top, and cover with a clean kitchen towel. Let proof in the fridge overnight.
If you want to accelerate this recipe, you can let the dough proof for 2 hours in an oven that has been turned on for 20 seconds and then off.
The morning after, preheat the oven at 250˚C (480˚F). Also, if you have it, preheat a pizza stone in the oven.
When the oven (and the pizza stone) is very hot, take the bread out of the banneton and put it on the pizza stone on the lower shelf. You can also put it on a baking tray with baking paper. We like to use a pizza paddle to do this.
Let cook for 25 min, then finish cooking for another 5 minutes on a higher shelf or with the grill function to darken the crust and create some extra flavour.
When the bread is cooked, turn off the oven and let the bread cool down win the oven with the oven door slightly open. Let cool down for about 30 min before serving.