To prepare the pâte fermentée, weigh or lightly spoon 5 ounces (about 1 cup) all-purpose flour into a dry measuring cup, and level with a knife. Combine 5 ounces all-purpose flour, 1/2 teaspoon yeast, and 1/2 teaspoon salt in the bowl of a stand mixer fitted with paddle attachment. Add 1/2 cup warm water, and beat at low speed until mixture is thoroughly combined. Add enough of the remaining 4 tablespoons flour, 1 tablespoon at a time, until dough just begins to pull away from sides of bowl. Increase mixer speed to medium, and beat for 2 minutes. Place dough in a small bowl coated with cooking spray. Place a piece of plastic wrap coated with cooking spray directly on dough. Refrigerate 48 hours or up to 3 days.
Remove pâte fermentée from refrigerator; let stand at room temperature 30 minutes.
To prepare the dough, place 3/4 cup warm water in the bowl of a stand mixer with dough hook attached, and sprinkle with 1 teaspoon yeast. Let mixture stand for 5 minutes or until bubbles form. Add pâte fermentée to yeast mixture; let stand 10 minutes. Weigh or lightly spoon 9 ounces all-purpose flour (about 2 cups) and 4 ounces bread flour (about 2/3 cup) into dry measuring cups; level with a knife. Add flours and 1 teaspoon salt to bowl; beat at low speed until flour is incorporated. Increase mixer to medium speed, and beat for 6 minutes. (Dough should form a ball.)
Place the dough in a large bowl coated with cooking spray; lightly coat dough with cooking spray. Cover with plastic wrap, and let stand in a warm, dry place, free from drafts, for 1 1/2 hours or until doubled in size. (Gently press two fingers into dough. If indentation remains, dough has risen enough.) Punch dough down, and divide into 2 equal portions. Knead each portion for 1 minute. Shape each dough portion into a 5-inch round by pulling the sides down, pinching and tucking them under the bottom center of dough, forming a smooth, taut surface on the top. Place dough rounds on a baking sheet sprinkled with cornmeal. Lightly coat surface of dough with cooking spray. Cover and let rise 1 hour and 15 minutes or until doubled in size.
Place a small ovenproof saucepan with 3 inches of water directly on the floor of oven. Preheat oven to 450°.
Uncover dough. Spray dough lightly with water. Make 4 (4-inch) cuts 1/4 inch deep in dough to form a grid across top of each loaf using a sharp knife. Bake at 450° for 10 minutes. Remove water pan from oven. Bake an additional 15 minutes or until bread is golden brown and sounds hollow when bottom is tapped. Cool on a wire rack. Cut each boule, crosswise, into 12 slices.