To make the starter, combine flour, water and yeast in a non-reactive bowl. Set aside to rest, covered, overnight.
For the dough, add water and flours to the starter. 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 fillings of choice 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.
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. 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 an 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.
To finish, brush the bread with olive oil and a sprinkle of fleur de sel.
Store bread, well-wrapped, at room temperature for three days or freeze for up to a month.