This dense, highly flavored bread is super-chewy thanks to bread flour's extra protein. It's wonderful served as is, but dip it in garlic- and herb-infused olive oil for a special treat.
To make the poolish: Weigh your flour; or measure it by gently spooning it into a cup, then sweeping off any excess.
Combine the flour, water and yeast, and set aside to rest, covered, overnight.
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.
To make the dough: Weigh or carefully measure your flours. 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.
Add the yeast, salt, and olive oil and knead the dough briefly, until it's a shaggy mass. Mix in the caramelized onions and olives and knead the dough until it's fairly supple but not necessarily elastic, about 3 minutes with a stand mixer or 5 minutes by hand.
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.
To shape the bread: 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.
Using a sharp knife or pair of scissors, 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, open up each slit by gently stretching the dough with your fingers, leaving behind holes that are at least 2" wide. This will ensure the leaf design is visible after the dough rises.
Allow the fougasse to rise until noticeably puffy, about 30 to 45 minutes.
Toward the end of the rising time, preheat the oven to 400°F. For extra-crispy crust, place a baking stone on the center rack.
Bake the fougasse for 20 minutes or until it turns golden brown. Remove it from the oven and transfer to a rack to cool.
Store bread, well-wrapped, at room temperature for three days; freeze for up to a month.
