Make sure you’re working with a starter that is 2-3 times as big as it was right after you fed it! Dump most of your starter into a large mixing bowl (leave a generous tablespoon in the jar, and feed it right now before you forget).
Pour 350ml of water into the mixing bowl, and whisk it all into a cloudy mess with no big blobs of starter left.
Measure out 500g of flour. I have been using 350g AP flour and 150g whole wheat, but all AP works fine too (that's what I first baked this recipe with). Whisk 2tsp salt into the dry flour or your bread will be sad and bland.
Stir the flour into the starter/water until it’s all evenly mixed together. You don’t want lumps or dry pockets anywhere. I recommend using a dough whisk (like the photo in the logo here), but if you don’t have one, a regular whisk will work for the first bit of flour, and after it’s too thick to whisk, you can switch to using your hands (oil your hands, or you will end up with dough gloves on).
Get a medium size mixing bowl and coat the inside with some oil. (Coat your hands with some oil, if you haven’t already.)
Scrape the dough out of the big bowl and roll it around in your hands to make an oily ball, then drop it into the medium bowl (it should be less than half full) and cover it. Plastic wrap with a towel on top is fine, but if the bowl has its own lid, even better.
Ignore the dough for 12 hours. Put it somewhere lukewarmish (65–75ºF or so), and just let it think about life for a long while. After 12 hours, when you look in the bowl, there should be a giant poofy dough in there. (If not, you’ll probably need to start over.)
Get your baking dish out and smear butter or oil around the inside. If you have a covered loaf pan, perfect. If not, any loaf pan will do.
Oil your hands again, and scrape the giant poofy dough out of the bowl. What I do is kind of hold the dough in both hands, and stretch the bottom side up over the top, repeatedly, in every direction. (I intend to do a video for this part. Email me if you want to see it!)
Pop any really big bubbles that come to the surface, but try not to squash all the air out of it. The dough ball should get tighter feeling as you do that… (don’t worry too much, it’s hard to screw this part up!)
When it feels like the dough has tightened up, squash it around into kind of an elongated oval, so it fits in the loaf pan nicely. The loaf pan should look about half full (maybe less).
Cover the loaf pan (plastic wrap & a towel, or a ready-made lid, whatever), and put it in the fridge for 8-12 hours. If you started step 1 in the morning, overnight is a perfect timer.
After the rest in the fridge, the loaf pan should look almost 3/4 full. If your loaf pan has an oven safe lid, move on to step 14. If you don't have a covered loaf pan, go to step 15.
COVERED LOAF PAN: Put your loaf pan in the oven, turn it on to 425°F, and set a timer for 30 minutes.
REGULAR LOAF PAN: Heat up a cast iron pan (or other oven safe pan) on the stove top. After you put your loaf pan in the oven (middle rack), slide the HOT HOT HOT pan in the bottom rack, and pour a cup of boiling water in the hot pan RIGHT before you close the oven. Trap as much of that steam inside the oven as you can! Then turn the oven on to 425ºF and set a timer for 30 minutes.
After 30 minutes, take the lid off the loaf pan, or take the pan of water out of the oven. The loaf should have nearly doubled in size, but still be very pale. Close the oven and restart your 30min timer.
After the second 30 minutes, the bread should be nicely browned, and over 200°F in the middle. If so, turn it out onto a cooling rack and cover it with a kitchen towel.
Store it in a plastic bag when it’s cooled down. That will keep the crust nice and soft.