1) Combine the starter, water, and 3 cups of the flour. Beat vigorously for 1 minute.
2) Cover, and let rest at room temperature for 4 hours. Refrigerate overnight, for about 12 hours.
3) Add the remaining ingredients: 2 cups of flour, sugar, salt, and sour salt, if you're using it. Knead to form a smooth dough.
4) Allow the dough to rise in a covered bowl until it's relaxed, smoothed out, and risen. Depending on the vigor of your starter, it may become REALLY puffy, as pictured; or it may just rise a bit. This can take anywhere from 2 to 5 hours. Understand this: sourdough bread (especially sourdough without added yeast) is as much art as science; everyone's timetable will be different. So please allow yourself to go with the flow, and not treat this as an exact, to-the-minute process.
5) Gently divide the dough in half.
6) Gently shape the dough into two oval loaves, and place them on a lightly greased or parchment-lined baking sheet. Cover and let rise until very puffy, about 2 to 4 hours. Don't worry if the loaves spread more than they rise; they'll pick up once they hit the oven's heat. Towards the end of the rising time, preheat the oven to 425°F.
7) Spray the loaves with lukewarm water.
8) Make two fairly deep diagonal slashes in each; a serrated bread knife, wielded firmly, works well here.
9) Bake the bread for 25 to 30 minutes, until it's a very deep golden brown. Remove it from the oven, and cool on a rack.