In a large mixing bowl (one that is easy to cover with plastic wrap) use a whisk or wooden spoon to mix the sourdough starter and water. Add in the flour and salt and use the wooden spoon to combine the ingredients until the dough is well incorporated and there are no pockets of flour left.
Cover the bowl with plastic wrap and let it sit on the counter at room temperature for at least 8 hours, or up to 24 hours. The dough should double in size and be full of bubbly air pockets.
When ready to bake, place a Dutch oven (with the lid on) in the oven and get it preheating to 450°F. You want the Dutch oven to be hot for baking. Once fully preheated, remove the pot from the oven and use an oven mitt to remove the lid.
Sprinkle flour over your hands and onto a piece of parchment paper (if not using parchment, then flour the bottom of the pot). The dough will be very sticky, but do your best to split it roughly in half; it doesn't have to be precise. Grab half of the dough and plop it onto the floured parchment. Lift the corners of the parchment to drop both it and the dough into the hot Dutch oven.
Bake for 30 minutes with the lid on, then remove the lid and bake for another 15-20 minutes with the lid off, or until golden brown. Remove the bread from the pot and let it cool completely before slicing. Repeat the process with the other half of the dough.