How to Make It
Preheat the oven to 350. On a rimmed baking sheet lined with parchment paper, thinly spread the rye flour. Toast for about 16 minutes, stirring every 4 minutes, until fragrant and just slightly darkened. Let cool.
In the bowl of a standing mixer fitted with the dough hook, combine the toasted rye flour with the 2 1/4 cups of bread flour, the water, cornichon juice, yeast, sugar and salt. Mix at low speed until well combined, about 1 minute. Increase the speed to medium and mix until the dough just starts to pull away from the side and bottom of the bowl, about 3 minutes; it will still be sticky and shaggy.
Meanwhile, lightly oil a large bowl. With oiled hands, shape the dough into a ball and transfer to the oiled bowl. Cover loosely with plastic wrap and let rise in a warm place for 1 hour; the dough will rise only slightly.
Invert the dough onto a lightly floured work surface. Flatten gently and fold the edge into the center. Invert and return the dough to the oiled bowl. Cover and let rise in a draft-free place until puffy, about 1 hour.
Lightly oil a 9-by-5-inch loaf pan. Invert the dough onto a lightly floured work surface and flatten into a 10-inch square. Roll up the dough to form a 10-inch log. Fit the loaf into the prepared pan, seam side down, tucking the ends underneath. Press the dough gently into the corners of the pan. Cover loosely and let rise in a warm place until the dough is just above the rim of the pan and no longer springs back to the touch, about 2 1/2 hours.
Set a pizza stone on the second-lowest rack in the oven and a cast-iron skillet on the bottom of the oven. Preheat the oven to 400 for at least 30 minutes. Place the loaf pan on the pizza stone. Add 1 cup of ice to the skillet and shut the oven. Bake for about 55 minutes, until the bread is lightly browned on top and the bottom of the pan sounds hollow when tapped; remove the skillet halfway through baking. Unmold the loaf onto a rack and let the bread cool before slicing.