Prepare baking dish: Cut a large piece of parchment and lay inside a large cast iron skillet or on a flat baking sheet.
Stir dry ingredients: Add flours, baking soda, salt, and sugar (if using) to a large mixing bowl. Stir together with a spoon.
Add wet ingredients: Pour sourdough discard and water into the dry mixture.
Mix: Stir until there are no dry patches left and a thick sticky mixture is made. Mixing with hands is helpful for this thick dough.
Shape: Place dough onto a work surface. Continue to mix and knead with hands for 1-3 minutes to check for dry spots and then use hands to form dough into a round shape. Place the round ball of dough in the center of the parchment. Cover with plastic wrap.
Ferment: Leave the dough covered at room temperature for 6-10 hours (or overnight) to ferment and rise. It will expand and spread into a large round of dough.
Preheat: After that time has passed, preheat oven to 425º F.
Score: Uncover dough. Use a knife to cut an x on the top of the dough across the dough that is about 1/2 inch deep in the center.
Optional step: Use a pastry brush to brush additional water on the top, in the cut x, and all around the sides of the bread. Sprinkle some additional sugar all over the crust on top of the moistened surface, once baked it will add a nice color and sugary crunch on the crust.
Bake: Place pan into oven and bake 30 – 35 minutes. The bread should have a golden brown top and a toothpick or knife inserted into the center should come out clean.
Cool: Allow the bread to cool for about 15-30 minutes.
Serve: Cut the bread into slices or wedges and serve.