Stir sourdough starter before measuring. Measure out 1 1/2 cups sourdough starter and bring to room temperature.
In large bowl, combine 2 cups flour, undissolved yeast, and salt. Gradually add water and starter to dry ingredients; beat 2 minutes at medium speed of electric mixer, scraping bowl occasionally. Beat 2 minutes at high speed. With spoon, stir in enough remaining flour to make soft dough. Knead on floured surface until smooth, about 8 minutes. Place in greased bowl, turning to grease top. Cover; let rise in warm place until doubled in size, 30 to 60 minutes. (With Rapid-Rise Yeast, cover kneaded dough and let rest on floured surface 10 minutes. Proceed with recipe.)
Punch dough down. Remove dough to floured surface; divide in half. Roll each half to 12 x 9-inch rectangle. Beginning at long end of each, roll up tightly as for jelly roll. Pinch seams and ends to seal. Taper ends by gently rolling back and forth. Place, seam sides down, on large greased baking sheet sprinkled with cornmeal. Cover; let rise in warm place until doubled in size, about 30 to 45 minutes.
With sharp knife, make 4 or 5 diagonal slashes (1/4 inch deep) across top of each loaf. Bake at 400oF for 35 or 40 minutes or until done. For crispy crust, spray loaves with water just before baking and every 5 minutes during the first 10 minutes of baking time. Remove from baking sheet; let cool on wire rack.
*The amount of flour needed varies according to the consistency of the starter.
TO MAKE ROUND LOAVES: Divide dough in half. Shape each half into 5-inch ball. Place on large greased baking sheet sprinkled with cornmeal. Cover; let rise in warm, draft-free place until doubled in size, about 30 to 45 minutes. With sharp knife, make 4 slashes (1/4 inch deep) in crisscross fashion on top of each loaf. Spray with water and bake as directed.
VARIATION: Replace 3/4 cup water with 3/4 cup yogurt, buttermilk, or flat beer.