1. To make the poolish: Combine the flour, water and yeast, and set aside to rest, covered, overnight.

  2. To make the dough: Add the 1/2 cup (113g) water and flours to the poolish, mix until shaggy, cover the bowl, and allow the mixture to rest for 20 minutes.

  3. Add the yeast, salt and olive oil, and knead the dough until it's fairly smooth but not necessarily elastic, about 3 minutes by machine, or 5 minutes by hand.

  4. Place the dough in a lightly greased bowl, cover the bowl, and allow the dough to rise, at room temperature, for 1 1/2 hours, gently deflating it and turning it over after 45 minutes. While the dough is rising, prepare the filling.

  5. To prepare the filling: Combine the butter and diced onions in a pan set over medium-low heat. Cook the onions for about 20 minutes, stirring every 5 minutes; they should be nicely caramelized. Transfer the onions to a bowl and cool to room temperature before using.

  6. To fill and shape the bread: Gently deflate the dough, and knead in the caramelized onions and olives; this will be a messy process, but just stick with it (literally!), and it'll get done.

  7. Shape the dough into a 12" x 6" oval, place it on a lightly greased baking sheet, and allow it to rise for about 30 minutes.

  8. Cut a single slit all the way through the dough, lengthwise down the center, stopping about 1" from each end. Follow up with three or four diagonal slits on each side of the center slit to create a leaf-like appearance. 

  9. Open up each slit by gently stretching the dough, leaving behind holes that are at least 2" wide. This will ensure the leaf design is visible after the dough rises.

  10. Allow the fougasse to rise until noticeably puffy, about 30 to 45 minutes.

  11. Preheat the oven to 400°F. For extra-crispy crust, place a baking stone on the center rack. 

  12. To bake the bread: Bake the fougasse for 20 minutes, or until it's starting to brown. Remove it from the oven, and transfer it to a rack to cool.

  13. Store bread, well-wrapped, at room temperature for three days; freeze for up to a month.