Place dough seam side down in a lightly greased large bowl. Allow to rise 1.5 to 2 hours at room temperature until about 50% larger.
Turn out dough onto a lightly floured surface (using pumpernickel flour) and gently deflate.
If using a Pullman loaf pan, grease with vegetable oil. Press dough out into a 12 by 10 inch (30 x 25 cm) rectangle with the long side towards you. Roll away from you to form a firm cylinder then pinch the seam and tuck the ends slightly. Place the rolled dough into the loaf pan seam side down and press gently into the corners.
If using a Dutch oven, sprinkle some corn meal on the bottom (optional). Shape the dough into a taut round and place seam side down into the center of the Dutch oven. There should be about an inch all the way around between the dough and the walls.
Cover loosely with greased plastic wrap and allow to rise 1.5 to 2 hours. If using a Pullman loaf pan, check that the dough is about 1/2 inch (1.25 cm) below the edge of the pan. Otherwise, press gently with your knuckle to ensure the dough springs back just a little.
Preheat your oven to 250 F/120 C and position a rack in the lower-middle of the oven with enough height to fit your cooking vessel plus a few inches.
Brush loaf with egg wash (or dust with pumpernickel flour) and slash about 1/2 inch (1.25 cm) deep. For a Pullman loaf, score in a long line from end to end, leaving 1/2 inch unslashed at each end. For the loaf shown in the photos, score a circle into the top instead. You can do whatever design you wish if not using this as a bowl.
Bake for 3 hours, turning the pan around halfway through.
Remove loaf from pan right away and allow to cool for 4 hours before serving.
