Mix together in a large bowl:
400 gm bread flour (I use 300-350 gm white, 100-50 gm whole wheat)
8-9 gm salt
¼ t dry yeast
(optional: any mixture of seeds, dried cranberries, raisins, chopped dried figs, walnuts, etc.)
Add:
300 ml water
Stir the water into the dry ingredients and form into a sticky ball.
Put a plate or something on top of the bowl, and set aside (in a warm room, if possible) for 18-24 hours. If you want to leave it for longer, put it in the fridge for a little while to slow down the rising.
Cut a piece of parchment paper and place it on a clean surface. Next, as if the dough were a square, pull one side up and over, then another side up and over, then another, then another. Finally, pick up the gooey mass and turn it over, smooth side up, and place it on the parchment paper. Put a big bowl over it (or a cloth, if you prefer), and let it rise for 1.5-2 hours. If the weather is cool, let the dough rise in the oven at 40ºC (104ºF).
I've since learned that flouring the surface is not necessary, but this is an old photo of the dough before rising:
When the dough has risen, preheat the oven and baking pan. See the risen dough below:
Place a Dutch oven, donabe, or other oven-safe covered pot into the oven and preheat to 230ºC (450ºF). When preheated, carefully remove the pot from the oven, take off the lid, and lift the risen dough into the hot pot. Put the lid back on, and put the pot on a low shelf in the oven.
Dough placed in the hot donabe pot:
Bake for 48 minutes at 230ºC (450ºF). Remove from oven. It should be done, but if you want to double-check, insert a thermometer. The internal temperature should be at least 96ºC (205ºF).
Cool on a rack before cutting.