Acme's Herb Slabs
For the dough:
By hand, combine the flour, salt, rosemary and yeast in a large bowl. Add the water and oil to the poolish, stir to loosen it, and pour it all into the flour mixture. Stir the mixture with your hand until it forms a rough dough. Turn it out onto your work surface and knead it briefly, without adding extra flour, until it is well combined. Cover the dough with a bowl and let it rest for 10 minutes to allow the yeast to rehydrate. Knead the dough, without adding extra flour, until it is very smooth, about 10 minutes.
Fermenting and turning the dough:
Place the dough in a container at least 3 times its size and cover it tightly with plastic wrap. Let the dough ferment until light and doubled in bulk, about 6 hours. Turn the dough (punch the dough) 3 times in 20-minute intervals, that is, after 20, 40, and 60 minutes of fermenting, then leave the dough undisturbed for the remaining time.
Shaping and proofing the dough:
Cut the dough in half. Round the pieces and let rest for about 20 minutes. Lightly press one piece of the dough into a rectangle. Loosely fold it into thirds like a business letter by folding the bottom short edge up and the top down. Place it seam side down on a couche (see note) and cover it with a flap of the couche. Repeat with the other piece. Let them proof for about 1½ hours.
Cover a peel or rimless baking sheet with a large piece of parchment paper. Remove the dough from the couche and gently press each piece into a 12 x 6 inch rectangle with your hands (the workers in the bakery use a small wooden ruler to get the dimensions just so). Press your fingertips deeply into the dough to stipple it all over. Move the rectangles of dough to the parchment paper and resquare them. Cover them with plastic wrap and let proof until very soft and well expanded, about 2 hours more. The total proof time is about 3½ hours.
Preheat the oven: About 45 minutes before the bread is fully proofed, arrange a rack on the ovens second-to-top shelf and place a baking stone on it. Clear away all racks above the one being used. Preheat the oven to 450°F.
Poke the dough all over with a toothpick or a skewer, pushing all the way through. If desired, just before baking, fill the oven with steam. Slip the breads, still on the paper, onto the hot stone and bake for 5 minutes. Carefully flip the breads over onto the stone and remove the paper. Continue baking until they are well browned, about 20 minutes more, rotating them after 10 minutes. Let the breads cool on a rack.
Note: Couche, meaning "layer" in French, refers to the heavy-linen cloth used to support breads, such as rolls and baguettes, as they are proofing.