-
To make the sponge: Measure the starter into a bowl. Add the water, coffee, or potato water; the pumpernickel, and onion. Stir together, cover, and let rest at room temperature overnight.
-
To make the dough: The next day, stir the oil, salt and molasses into the sponge. Stir in the flour 1 cup at a time, until the dough comes together.
-
Turn the dough out onto a lightly floured surface and knead, adding only enough additional flour to keep it from sticking to your hands.
-
Once the dough has come together, shape it into a ball (boule). Place the boule smooth side down in a flour-dusted or lined brotform or bowl.
-
Cover and let rise in a draft-free spot until puffy. This will take anywhere from 1 to 3 hours depending on the temperature of the space where the dough is rising and the strength of your starter.
-
About an hour before the boule is finished rising, preheat the oven to 425°F with a baking stone on the center rack. Place an empty cast-iron skillet on the lowest rack.
-
Turn the boule, smooth side up, very gently onto a baker's peel dusted with semolina or cornmeal, or onto a piece of parchment. Use a lame or very sharp knife to make several slashes across the top of the loaf.
-
Transfer the boule to the hot stone, and add steam to the oven: Pour about 1 cup of boiling water into the cast iron frying pan. Steam will billow from the pan upwards to envelop the baking bread; be sure to wear good oven mitts to shield your hands and arms. Quickly close the oven door to trap the steam.
-
Bake the boule for 40 to 45 minutes. It's done when the center temperature is 200°F when measured with a digital thermometer. Remove the boule from the oven and cool it on a rack before slicing.
-
Store any leftover bread, loosely wrapped, at room temperature for several days; freeze for longer storage.
