How to Make It
In a large bowl, cover the cereal with 3 inches of water. Cover the bowl with cheesecloth or a kitchen towel and let stand at room temperature for 48 hours. Drain through a cheesecloth-lined sieve; reserve 11/2 cups of the soaking water.
Meanwhile, in a large bowl, mix the all-purpose flour with the yeast and the water until the dough starts to come together. Turn the dough out onto a work surface and knead until a smooth ball forms, about 3 minutes. Wipe out the bowl and return the dough to it. Cover the bowl with plastic wrap and refrigerate the dough for 48 hours. This is the pâte fermentée.
In the bowl of a stand mixer fitted with the dough hook, combine the bread flour, sea salt, pâte fermentée, reserved soaking water and 2 cups (1 pound) of the soaked cereal; there may be some soaked cereal left over. Knead at low speed until the dough starts to come together, about 3 minutes. Cover with a kitchen towel and let stand for 20 minutes. Knead again at low speed for 5 minutes, until the dough is moist and smooth. Transfer the dough to a lightly greased bowl, cover and let rise at room temperature until doubled in bulk, about 2 hours.
Line 2 large bowls with kitchen towels or dust generously with flour. Divide the dough into 2 equal pieces and form each into a ball. Set the balls seam side up in the prepared bowls. Cover with kitchen towels and let stand at room temperature until doubled in bulk, about 2 hours.
Preheat the oven to 450°. Line 2 baking sheets with parchment paper. Place another baking sheet on the bottom of the oven and pour in 2 cups of water. Turn the dough rounds out onto the prepared sheets and bake for about 40 minutes, rotating and shifting the sheets halfway through baking, until the loaves are deep golden and crisp. Transfer to a rack to cool.