In a large bowl, stir together 1⁄2 cup warm (90°) water , the starter, and 1 cup flour until smooth. For sourest flavor, cover and let stand in a warm place until bubbly and sour smelling, 12 to 24 hours, before proceeding. (The longer you let it ferment , the more pronounced the sour flavor will be in the finished bread.) Soften yeast in 1⁄4 cup warm (90°) water; stir into sourdough mixture with salt and sugar.
Using your method of choice, mix in remaining flour and knead:
By hand: Stir in enough flour to form a kneadable dough , about 2 cups. Turn dough out onto a lightly floured work surface and knead until smooth and elastic, 12 to 15 minutes; add as little flour as possible to keep dough from sticking. Put dough in a greased bowl; turn over.
In a stand mixer fitted with a dough hook: Mix in enough flour to form a somewhat stiff dough, about 2 cups. Beat on high speed until dough pulls cleanly from inside of bowl, about 8 minutes. If dough still sticks or feels sticky, add 1 tbsp. flour at a time until dough pulls free and isnt sticky. Leave in bowl.
In a food processor: Put 21⁄4 cups flour in processor with metal blade. With motor running, pour sourdough yeast mixture into feed tube. Process until dough forms a ball and pulls from container, then process 45 seconds more. If dough feels sticky, add 1 tbsp. flour at a time and mix in short bursts. Put dough in a greased bowl; turn over.
Cover bowl with plastic wrap and let rise in a warm (about 80°) place until doubled in bulk, 1 to 2 hours.
Gently punch down dough. Turn out onto a lightly floured work surface and knead gently just until smooth. Roll with your hands into a 3 1⁄2- by 12-in. log. Generously sprinkle a piece of stiff cardboard with cornmeal; set dough on cornmeal. Cover lightly with plastic wrap and let rise in a warm place until puffy, 10 to 30 minutes.
Meanwhile, put a 12- by 15-in. baking sheet on lowest oven rack, then preheat oven to 400°.
With a flour-dusted razor blade or sharp knife, cut several 3⁄4-in.-deep diagonal slashes on loafs top. Slip loaf off cardboard onto hot baking sheet, keeping slashed side up. Mist bread all over with water from a spray bottle.
Bake for 5 minutes, and then mist again with water. Repeat after 5 more minutes, then bake until deep golden, 25 to 30 minutes more. Let cool completely on rack.
Quick Tip: Although its gratifying to make your own sourdough starter, you can also order one online from amazon.com; we like Goldrush Sourdough starter.
Variation: To form the dough into a round loaf, shape it after you have kneaded the dough in step 4, tucking the edges under and pushing them back up into the bottom of the dough (the top of the round should be slightly stretched). Put on a buttered baking sheet, cover, and let rise as directed, then bake as for a baguette.