Combine the flour and salt in a large mixing bowl and whisk together. Make a well in the dry ingredients and pour in 2-1/4 cups of the water and the sourdough starter. Mix until a shaggy dough forms, adding the additional 1/4 cup of water as needed.
Once the dough is fully hydrated, let it rest five minutes in the bowl. Return to the bowl and knead for 1-2 minutes directly in the bowl. Resist the urge to add any flour. The dough will be sticky so after 1-2 minutes allow it to rest for several more minutes. Finally, you should be able to knead it for 2-3 more minutes without it sticking heavily to your hands.
Once the kneading is complete, cover the bowl and leave to ferment in a warm space in your kitchen for 4-6 hours or until nearly doubled in bulk. Divide the dough into two halves and punch down each half.
Grease and lightly flour a baking sheet. Turn each half of dough into a boule by stretching the dough upon itself into a circle, turning the dough as needed to make an even round ball all of the way around.
Once the dough ball is tightly shaped, place it seam-side down onto the baking sheet, leaving room for two loaves. Repeat with the second portion of dough. Sprinkle the loaves lightly with flour and over with a towel or plastic wrap. Allow to rise two-to-three hours or until the dough has grown by about 75%.
During the last 30 minutes of rise time, preheat the oven to 400 degrees. Uncover the dough just before placing in the oven and give it a few slashes with a sharp knife. Move the loaves to the oven and bake for 35-40 minutes or until golden brown and the bottoms sound hollow when thumped.
Move boule bread loaves to a cooling rack and allow to cool for at least 30 minutes before slicing.
Notes
To get a good rise out of this sourdough boule bread recipe, begin with an actively bubbling sourdough starter. Once the dough is blended, the rise time can vary depending upon the air temperature. On a cool day, it may take a bit longer to achieve its loft.