Test your sourdough starter by placing a small spoonful in a glass of water. If it floats to the top, it's ready.
In a large bowl, mix together the sourdough start, warm water, salt, and half of the flour. Stir until smooth.
Add the remaining flour and stir until the dough starts to pull away from the sides of the bowl.
Cover with plastic wrap or a lid and let sit at room temperature for 30 minutes. Fold one corner of the dough over to the other side. Rotate the bowl a quarter turn and fold over each of the corners.
Repeat 3-5 times. The bread dough will become much more smooth and elastic after it has rested and been stretched a few times. (Tip - if the dough seems too wet, flour your hands before folding & stretching the dough. If it seems too dry, wet your hands with water.)
Form the dough into a ball shape in the bowl and cover with plastic wrap or a lid. Refrigerate for at least 12 hours, but up to 24.
The next morning, take the dough out of the refrigerator and let it sit at room temperature for 1-3 hours. Form it into a ball by tucking the ends under and forming a smooth top.
Place the dough ball on a piece of parchment paper. Cut slits in the top of the dough. Preheat your oven and a 4-quart Dutch oven to 450°.
Grab the edges of the parchment and gently place the parchment and dough into the hot pan. If desired, slip a couple of ice cubes between the parchment and pan. (This gives the bread a nicer rice and chewier texture.)
Place the lid on the pan and bake for 35-40 minutes at 450 degrees.
Take the lid off the pan. The bread should have risen to almost the top of the pan, and be barely golden brown.
Continue baking for 10-20 more minutes, or until internal temperature is 200° and the crust is as brown as you like it.
Grab the edges of the parchment to remove the bread. Place it on a cooling rack. Let it cool for at least 20 minutes, but it slices easier after it has completely cooled.