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.
What makes the sour in sourdough bread? It's a combination of lactic and acetic acids, created as the dough rises and ferments. Refrigerating the dough encourages the production of more acetic than lactic acid; and acetic acid is much the tangier of the two. Thus, sourdough that's refrigerated before baking will have a more assertive sour flavor.
Adding citric acid gives your bread an extra hit of "sour;" but don't be tempted to go beyond about 5/8 teaspoon in this recipe. A good rule of thumb for ultimate sourness, without too much deterioration of the crust and bread's structure, is 1/8 teaspoon sour salt for each cup of flour used.
Depending on the thickness of your sourdough, you may need to add additional water or flour during the kneading stage. Your goal is a soft, elastic (but not sticky) dough.