In a large bowl, combine the white flour, wheat flour, and salt, and whisk to combine. In a small bowl, whisk the starter and water until the starter is fully dissolved. Pour the starter mixture into the flour, and use a spatula to mix. Cover the bowl loosely with a clean towel, and let the dough sit at room temperature for a half hour.

Turn the dough, pulling it off the sides of the bowl and stretching it over the center as you turn; work it as little as possible. Cover loosely and let rest for 30 minutes before turning the dough again. Repeat this process for 2.5 to 3 hours, mixing the dough approximately five times. The dough will be ready when it easily peels off the bowl and stretches over itself when shaping.

Shape the dough into a ball then set seam side down.

Preheat the oven to 500 F.

Preheat a cast-iron ovenproof Dutch oven with a tight-fitting lid in the oven. Carefully remove the lid and transfer the dough on the parchment into the pot. Slash the top of the dough with a serrated knife to allow steam to escape. You can make a cross or another decorative design. Cover the pot immediately and place the pot in the oven.

Bake the bread for 35-40 minutes with the lid on the Dutch oven. Carefully remove the lid and tear off any excess parchment. Bake for another 10 to 15 minutes with the lid off, until the crust is a very dark brown. Remove the loaf from the pot. Cool the loaf on a wire rack. The loaf will continue to bake as it cools, so wait an hour before cutting into it.

Tip: Sourdough starter can be made from scratch or - better yet - acquired from a friend. Any good "bread head" will gladly share their bounty. Once you have established or acquired your starter you will need to "feed" it on a regular basis. The process is quite simple and can be done as little as once a week if started is stored in the refrigerator.