The night before, dissolve the yeast in the water for the Poolish, then mix in the all-purpose flour. Cover and let it ferment at room temperature for 12-16 hours, or overnight. Once the Poolish has bubbles breaking on the surface and has started to wrinkle, it’s ready.
Transfer the Poolish to the bowl of a stand mixer fitted with a dough hook. Add water, salt, yeast, and all-purpose flour, and knead the dough on slow speed for 2-3 minutes. Cover and let it rise for two hours, performing a stretch-and-fold at the one-hour mark.
Divide the dough into three equal pieces. Cover and let them rest for about 20 minutes. Shape each piece into a baguette about 12 inches long, then cover and let them rise for 60 to 90 minutes.
Score the baguettes and bake in a preheated oven at 450°F (232°C). Place a tray of water below the baguettes while baking to create steam in the oven. Bake for about 25 minutes. Remove from the oven, let cool, and serve.