Combine bread flour, water, and yeast (rehydrated with 1 Tbsp. water from the 3/4 cup) for the poolish starter in a measuring cup or bowl until the mixture looks like lumpy pancake batter. Cover poolish with plastic wrap and let it rest at room temperature for 3–4 hours. Surface will be bubbly. Refrigerate poolish overnight, then let poolish come to room temperature (about 1 hour) before using it in bread dough.

Whisk together 1 1/2 cups all-purpose flour, rosemary, and salt for the dough in a large bowl. Add yeast (rehydrated with 1 Tbsp. water from the 1/4 cup). Stir remaining water into poolish starter. (It will be very thin.) Using a wooden spoon, stir poolish and olive oil into flour mixture until blended. (Dough will be very wet.) Scrape dough onto well-floured surface and turn with bench knife, adding more flour (from remaining 1/2 cup) sparingly, just until dough can be worked with hands. Turn a bowl upside down over dough and let dough rest for 10–15 minutes to develop gluten.

Knead dough until smooth but still slightly sticky. (Do not add more flour to work surface; if needed, flour hands.) Knead 10–15 minutes more by hand (or 8 minutes by stand mixer on medium speed with a dough hook), or until dough passes "window pane" test (see photo, Issue 73, page 20).

Place dough in an oiled bowl, turning to coat. Cover dough with plastic wrap; let rise in a warm place for 1 hour. Degas dough slightly by tri-folding and flipping it over in the bowl; cover dough again and let it rise for 1 more hour.

Shape dough into boule (see photos 1 & 2, Issue 73, page 24). Transfer to a banneton (see Issue 73, page 24) or parchment-lined inverted baking sheet. Cover boule with plastic wrap coated with nonstick spray and let rise 1 hour.

Preheat oven to 475°, with baking stone (or another inverted baking sheet) placed on middle rack. If using banneton, flip boule onto back of cool parchment-lined baking sheet so seam is on the bottom. Slash boule 1/4- to 1/2-inch deep several times across top in design of choice, using a straight-edge razor or serrated knife.

Mist boule heavily with hot water. Immediately slide boule and parchment onto preheated baking stone (or hot inverted baking sheet). Mist inside of oven with 10 sprays of water; close oven door. Wait 30 seconds and repeat. Reduce heat to 450° and bake boule 10 minutes. Reduce heat to 425°; bake boule an additional 10 minutes. Finally, reduce heat to 375° and bake boule 20 more minutes. Remove boule from oven; cool bread on rack 45 minutes.