In the bowl of a stand mixer fitted with the dough hook, add the starter, water, yeast, sugar, salt, and the 2 ½ cups whole wheat flour. Mix on medium-low speed to combine, while gradually adding the bread flour as the ingredients begin to incorporate. Scrape down the sides as necessary and continue mixing on medium-low until a dough ball forms. On low speed, knead the dough for 7-8 minutes until a smooth, elastic ball forms (test the elasticity by poking a finger in the ball of dough; if it springs back, you are done kneading. If a dimple remains, keep kneading).

Transfer the dough ball to a clean bowl that is lightly coating with cooking spray. Cover loosely with a tea towel or plastic wrap and let rise until double in size; about 90 minutes.

Punch the risen dough ball down and divide it into two even sections. Shape it into a round ball and transfer to your well-floured 10’’ Banneton bread proofing basket.

Let the shaped loaf rise until double in size and puffy; about 1 hour.

Preheat your oven to 425? towards the end of the rising time.

Once the loaf has risen, carefully place a baking sheet pan on top of the bread proofing baskets and gently flip the loaf onto the pan. If you are using loaf pans or a cast iron pot, place the risen loaves directly into the oven in these containers.

Spray the sides of the oven with a little lukewarm water and bake the loaves for 25-30 minutes until they are a deep golden brown. Remove from oven and cool for 5 minutes before transferring the loaves to a wire rack to cool completely before slicing with a serrated knife.