The evening before mix together the yeast (0.2g) and water (90g). Whisk together. Add the bread flour (150g) and mix well. It'll be uneven. Cover and let rest at room temperature overnight.
The Dough
The next morning, the biga should've doubled. Or more.
Mix the water (300g), biga (all), dry yeast (5g) well. Add the flour (315g) and salt and mix on medium for 5-6 minutes until incorporated. Add 50-60g more of water a little at a time with the mixer on low-to-medium until all the water is added
Oil the square pan and add the dough. Cover with plastic wrap or put the lid on. Whichever you've got.
30 minutes later: Wet hands and do stretches and folds. Cover.
30 minutes later: 2nd stretch and fold
30 minutes later: 3rd stretch and fold
30 minutes later: 4th stretch and fold.
30 minutes later: Dough should've doubled in volume by now. If not, let ferment a little longer. Could be that your kitchen is a bit cool.
Generously flour work surface and the top of the dough. Turn the dough out. With wet or oiled hands, fold the dough onto itself so that you're folding the unfloured halves together.
Using a scraper or bench knife, divide the dough into to equal parts. Flour the tops and sides.
If you have a couche, bunch it up in the middle and transfer the two doughs to each side. I don't have a couche, so I use a bit of well-floured parchment on a half-sheet pan.
Cover with a towel and let proof for 30-60 minutes.
Heat oven to 500℉ with a baking stone on a rack on the middle. On the rack below you should have a shallow pan with hot water.
Poke the dough. Springs back immediately and completely, let rest a bit more. Should come back kinda slowly and still have an indentation.
If you've used a couche, now you need to transfer the loaves to parchment. Ideally, though, the parchment should be not a lot larger than each loaf. Flip the loaf onto this small parchment and slide into the oven and baking stone.
Bake at 500℉ for 10 minutes.
Remove the tray with the water, reduce the temperature to 475℉, bake for an additional 15 minutes.
Notes
This is sticky. Sticky sticky. Ultra sticky. Oil your hands well.