Whisk the water and yeast in a glass or metal bowl. Add 1 cup bread flour and whisk until smooth. This is the biga. Cover the biga with plastic wrap and refrigerate overnight (or up to 16 hours).
Bring the biga to room temperature for about 1 hour. It will be thick and foamy.
In a stand up mixer attached with a bread hook, mix biga, remaining 2 cups flour, salt, and sugar on low speed for 1 minute. Increase speed to medium and mix for 5 minutes. . If the dough is not wet, sticky, and falling from the hook, add 2 tsp. water at a time (I have made this twice and haven’t had this problem!) Transfer to a bowl sprayed with Pam. Pull the dough up and over the top until it is smooth. Cover with plastic wrap and set in warm, draft free area for 1 ½ to 2 hours or until doubled in size.
When the dough has doubled it’s ready to go in the pan. Drizzle 3 Tbsp. olive oil in bottom of a 9 x 13" pan. Smear with hands. Press dough evenly into the pan, cover with plastic and let rise 1 hour. Preheat oven to 400°. Top dough with rosemary (blanch if you want). Coat fingers with Pam and gently make dimples in dough (not too many or it will collapse. Drizzle dough with olive oil and kosher salt. Bake for 25-30 minutes or until golden and sides pull away from the pan. Let cool 5 minutes before serving.