Special equipment: 2 pans, either jelly roll, 1/4 sheet, or 9-inch square metal
To make the poolish: Whisk the whole wheat flour, steel-cut oats, water and yeast in medium bowl. Cover with plastic and set aside at room temperature for 12 hours or overnight.
To make the dough: Whisk the all-purpose, whole wheat, and oat flour, flaxseed, yeast, and salt in a large bowl. Make a well in the center and pour in the poolish, water, and olive oil. Stir the wet ingredients into the flour with a wooden spoon to make a very sticky loose dough. Cover with a kitchen towel and let stand for 30 minutes.
Turn the dough out of the bowl onto a well-floured work surface or prep mat. Coat your hands with flour and press dough into a 12 by 8-inch rectangle (long side towards you). Using a bench scraper, fold the dough as you would a business letter. (The dough is very wet and this may seem odd, but just move quickly with the scraper and fold 1 end of the dough over the other. Make sure you brush any raw flour from the surface of the dough before you fold over the second end). Spray with cooking spray and cover with a kitchen towel. Let stand 30 minutes.
Fold the dough again like a letter, and rest for another 30 minutes.
Preheat the oven to 425 degrees F. Divide the olive oil among 2 pans and swirl to coat the pan. Divide dough in half and transfer a piece to each pan, turn dough over to coat both sides with oil, then press dough evenly into the pans. Scatter the garlic, thyme, and rosemary over the top and press the toppings into the dough with your fingers. Sprinkle with sea salt and scatter the cheese on top. (See Cook's Note.)
Bake until golden brown, about 20 to 25 minutes. Slip focaccia from the pans and cool on a rack.
*The focaccia dough can be tightly wrapped and refrigerated at this point for up to 3 days. Bring to room temperature for about 2 hours before baking.
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