In large bowl using wooden spoon, stir starter with 3/4 cup of the warm water until combined. Stir in 1 1/2 cups of the flour until smooth; cover with plastic wrap. Let rise at room temperature for 12 hours. (Make-ahead: Let rise for up to 24 hours.)
Stir in remaining warm water. Stir in 1 3/4 cups of the remaining flour and the salt. Turn out onto floured work surface. Knead dough, dusting with as much of the remaining flour as needed to prevent sticking, until smooth and elastic, about 8 minutes. Place in greased bowl, turning to grease all over; cover with plastic wrap. Let rise in warm draft-free place until tripled in size, about 2 hours.
Punch down dough; turn out onto floured work surface. Knead dough about 10 times to remove air bubbles; cover with tea towel. Let rest for 10 minutes.
Line 12-cup bowl with tea towel; dust with some of the remaining flour. Shape dough into ball. Place, seam side up, in bowl; cover with clean tea towel. Let rise in warm draft-free place until doubled in size, about 1 1/2 hours.
Dust rimless baking sheet with cornmeal. Place dough, seam side down, on pan. Whisk egg with 1 tbsp water; brush over dough. Using serrated knife or kitchen scissors, cut three 1/2-inch deep slashes in top. Sprinkle with 1 tsp of the remaining flour. Bake in 425°F oven until golden and loaf sounds hollow when tapped, 35 to 40 minutes. Let cool on rack for 45 minutes.
In 8-cup container, stir together 2 cups all-purpose flour, 1 cup warm water (100°F) and 1/4 tsp active dry yeast; cover with plastic wrap. Let rise at room temperature until tripled in size, about 8 hours. (Make-ahead: Let rise for up to 24 hours.) Refrigerate for 3 days. Stir in 1/2 cup each all-purpose flour and water; cover and refrigerate for 3 days. Makes 1 batch.
Tip from The Test Kitchen: Feeding the Sourdough Starter with flour and water is essential to keeping the yeast active. To feed, stir in 1 1/4 cups all-purpose flour and 3/4 cup water or milk. Refrigerate for at least 2 days or for up to 1 week. Repeat feeding once a week until ready to use.
Change it up: Sourdough Loaves
Follow first 2 paragraphs of White Sourdough Boule recipe as directed. Divide dough in half. Using floured hands, press into two 1-inch thick circles. Fold each circle, 1 1/2 inches from edge farthest from you, pressing with hands; repeat folding, pressing each into torpedo shape. Pinch seams to seal. Dust rimless baking sheet with cornmeal. Place dough, seam side down and 4 inches apart, on pan; cover with clean tea towel. Let rise until doubled in size, about 1 1/2 hours. Using scissors, cut 3 slashes crosswise on surface of each. Whisk egg with 1 tbsp water; brush over dough. Sprinkle with 1 tsp of the remaining flour. Bake in 425°F oven until golden and loaves sound hollow when tapped, about 25 minutes. Let cool on rack for 45 minutes.
Get the recipe for our Cultured Butter.