1. FOR THE BIGA: Combine flour, water, and yeast in large bowl and stir with wooden spoon until uniform mass forms and no dry flour remains, about 1 minute. Cover bowl tightly with plastic wrap and let stand at room temperature (about 70°F) overnight (at least 8 hours and up to 24 hours.) Use immediately or store in refrigerator for up to 3 days (allow to stand at room temperature 30 minutes before proceeding with recipe.)

2. FOR THE DOUGH: Stir flour, water, and yeast into biga with wooden spoon until uniform mass forms and no dry flour remains, about 1 minute. Cover with plastic wrap and let rise at room temperature for 15 minutes.

3. Sprinkle 2 teaspoons salt over dough; stir into dough until thoroughly incorporated, about 1 minute. Cover with plastic wrap and let rise at room temperature for 30 minutes. Spray rubber spatula or bowl scraper with nonstick cooking spray; fold partially risen dough over itself by gently lifting and folding edge of dough toward middle. Turn bowl 90 degrees; fold again. Turn bowl and fold dough 6 more times (total of 8 turns.) Cover with plastic wrap and let rise for 30 minutes. Repeat stretching, folding, and rising 2 more times, for total of 1 hour.

4. Adjust oven rack to upper-middle position, place baking stone on rack, and heat oven to 500°F. Cook pancetta in 12-inch skillet over medium heat, stirring occasionally, until most of fat has rendered, about 10 minutes. Using slotted spoon, transfer pancetta to paper towelÝ-lined plate. Return skillet to medium heat, add onion and water; cook, stirring frequently, until onions are soft and beginning to brown, about 12 minutes. Remove pan from heat and set aside.

5. Transfer dough to lightly floured counter, being careful not to deflate completely. Lightly dust top of dough with flour and divide in half. Gently shape each piece of dough into 5-inch round by gently tucking edges underneath themselves. Coat two 9-inch round cake pans with 2 tablespoons olive oil each. Sprinkle each pan with 1/2 teaspoon kosher salt. Place round of dough in pan, top side down, slide dough around pan to coat bottom and sides, then flip over. Repeat with second piece of dough. Cover pans with plastic wrap and let rest 5 minutes.

6. Using fingertips, press dough out toward edges of pan, taking care not to tear it. (If dough resists stretching, let it relax for 5 to 10 minutes before trying to stretch again.) Using dinner fork, poke entire surface of dough 25 to 30 times. If any large bubbles remain on surface or sides of dough, pop with fork to deflate. Sprinkle pancetta, onions, and oregano evenly over top of dough. Let dough rest in pan until slightly bubbly, 5 to 10 minutes.

7. Place pans on baking stone and lower oven temperature to 450°F. Bake until tops are golden brown, 25 to 28 minutes, switching position of pans halfway through baking. Transfer pans to wire rack and let cool for 5 minutes. Remove loaves from pan, return to wire rack, and brush tops with any oil remaining in pan. Let cool 30 minutes before serving.

Makes two 9-inch round loaves