9-10ouncewater(warm water if it's not summer (1 1/8 to 1 1/4 cups water)
To make the poolish, mix together the 3.4 oz flour, 3.6 oz water, and 1/12 tsp yeast in a small mixing bowl. The dough will be a sticky paste. Cover with plastic wrap and let rise at room temperature for 3-4 hours, until the mixture is foamy. Refrigerate overnight.
Remove the poolish from the fridge 1 hour before you want to make the dough.
Combine poolish, flours, salt, yeast, and 9 ounces of water in the bowl of a stand mixer. Mix with the dough hook for about 6 minutes on medium. Dough will be soft, stretchy, and tacky, and will pass the windowpane test. Place in a large, oiled bowl, cover with plastic wrap, and let rise at room temperature for 2 hours, until the dough doubles in size.
Knead the dough gently for 1 minute. I usually just do this in my hands, folding the ball of dough on itself several times, instead of on the counter. Return the dough to the bowl and let the dough rise for another 2 hours, until the dough has doubled in size.
Gently transfer the dough to a floured surface. Cut into 3 equal pieces, being careful not to deflate the dough.
Shape each piece into a baguette. Here, you want to be sure to create good surface tension so that the dough stays in a baguette shape and doesn't spread outward as it rises. Pat the dough into a rectangle. Fold the bottom third of the dough up to the center as if you're folding a letter. Press the edge of the dough into the center to seal. Fold the other edge of the dough over the top and use your hand to press the seam against the counter to seal and create surface tension. Repeat with the other 2 pieces of dough to give the first a chance to relax.
Next, stretch each piece of dough out from the ends. Create a crease down the center of the dough, and fold like a letter as described above, sealing the dough in the center. Pull the top of the dough over and use your hands to seal the crease against the counter to create surface tension. Gently roll the loaf out on the counter to smooth the seal and extend the length to the length of your baking sheet. Repeat with the other 2 pieces of dough.
Line a large baking sheet with parchment, then sprinkle with cornmeal. Place each baguette on the parchment and cover with plastic wrap. Let the baguettes rise for 50-60 minutes, until the dough has grown by about 50%.
Place a metal pan on the bottom rack of your oven, and preheat the oven to 500F. Once the oven is at temperature, score your baguettes with a sharp knife. (I don't have any good advice on this, except to try to get your cuts both long and somewhat deep.) Place the sheet pan on the middle rack of your oven. Next, carefully pour 1 cup hot water into the heated metal pan in the oven, being careful to not get steam on yourself or on the glass window of your oven door if it has one. Close the door for 30 seconds. Use a mister to spray water on the oven walls (it's okay if it gets on the bread too), then close the door for 30 seconds. Spray again, close the oven for 30 seconds, then spray again.
Turn the oven down to 450F and bake for 10 minutes. Rotate the pan for even baking, and bake another 8-12 minutes, until the bread is a golden brown.
Remove the bread from the oven and cool for at least 40 minutes before slicing.