Dissolve the yeast in the warm water and let it proof. Measure the flour into a mixing bowl, add the salt, and blend well. Add the oil, and then, gradually, the buttermilk. Mix with the hands or in an electric mixer with a dough hook until the dough comes off the bowl.
Turn out onto a lightly floured board and knead for 10 minutes, until the dough is smooth and resilient. Remove to a buttered bowl and turn to coat the surface with butter. Cover and let rise until doubled in bulk.
Punch the dough down, knead for 3 minutes, and let rise once more. Punch down again, then, using both hands, gather the dough into a big circular package, draw the top together to close it, and pinch the ends together.
Turn the dough over, and set it, pinched-end side down, on a baking sheet sprinkled with cornmeal, and let rise until doubled in bulk. (Cornmeal may be sprinkled on the top, too, for an extra accent.)
Slash the top in three places and brush with the egg wash.
Broil at 375 degrees for 20 minutes, then switch to “bake” for 25 minutes, or until the loaf sounds hollow when tapped with the knuckles. (If your broiler has no setting, place the bread as far as possible from the unit and watch carefully. You may have to switch from “broil” to “bake” more quickly.)
Remove the loaf from the baking sheet and let it rest directly on the oven rack for a few minutes to brown the bottom.
Cool on a rack.