Making the poolish

1. In a mixing bowl, whisk together the rye and wheat flours and yeast and gradually add in the water. Stir with a wooden spoon or Danish dough whisk until the mixture is completely blended. Cover the bowl with plastic wrap and let the poolish rest overnight at room temperature (70 degrees F.) for 8 to 12 hours.
   

Making the final dough

1. In the bowl of a stand mixer, whisk together the flours, yeast and salt. Add in the poolish and the water and mix on low speed using the dough hook just until blended. Increase to medium speed and mix the dough for about 5 to 10 minutes, or until the dough begins to pull away from the sides of the bowl. It should feel elastic at this point. Add in the walnuts on low speed.
2. Shape the dough into a ball and transfer to a lightly oiled large bowl. Cover the dough with plastic wrap and let it proof for 1 hour at room temperature.
3. Transfer the dough to a lightly floured work surface. Shape into a rough ball, cover with plastic wrap or a kitchen towel and let it bench rest for 15 minutes.
4. Lightly flour a lined banneton (proofing basket) with white rice flour.
5. Shape the dough into a tighter round and place it in the banneton basket seamside up.
6. Cover with plastic wrap again and let it proof for 1 hour.
   
7. While the loaf is proofing, preheat the oven to 450 degrees F. with the cloche on the bottom rack. Allow the oven and the cloche to preheat for at least 45 minutes.
8. Carefully remove the preheated cloche from the oven using heavy oven mitts. Place the lid on a heavy towel or pot holder to keep it from cracking from the temperature change. Put the bottom on another towel or pot holder. Sprinkle the bottom of the baker with cornmeal.
9. Flip the loaf from the proofing basket to the bottom of the baker.
10. Using a lame or serrated knife, score the loaf in the pattern of your choice.
11. Transfer the bottom of the baker to the oven and place the lid on top. Let the loaf bake for 30 minutes with the lid on; then remove the lid and let it bake another 15 to 20 minutes or until it is a deep brown and sounds hollow when thumped on the bottom.
12. Remove the loaf from the oven and transfer to a wire rack to cool completely before slicing.