In the mixing bowl of a heavy-duty electric mixer, sprinkle the yeast over the water. Let it sit until the yeast dissolves and the water looks milky. Add the flours, salt, and walnuts. With the paddle attachment, mix on low speed until a rough dough forms. Change to the dough hook and knead on medium speed until the dough pulls away from the bowl to form a ball. You may need to add a small amount of flour or water to get the right consistency.
Turn the dough out into a lightly oiled bowl and cover with plastic wrap. Set in a warm spot (about 75°F) until doubled in bulk, 2-1/2 to 3 hours.
Line a basket or bowl with a kitchen towel or a piece of cotton fabric and sprinkle it lightly with flour. Turn the dough out onto a lightly floured work surface, knead it a few times, and shape it into a round. Put it in the basket, bottom up, and fold the ends of the towel over it. Cover with a large plastic bag (like a kitchen garbage bag). Set in a warm place and let rise until doubled in bulk, about 1-1/2 hours.
Set one oven rack in the lowest position; put a large, shallow metal pan on that rack. (For safety reasons, do not use a glass pan for this, as it may shatter.) Set the second rack just above that and position the baking stone on it. Heat the oven to 425°F for at least 45 minutes. Just before you put the bread in the oven, bring about 2 cups of water to a boil.
Lightly dust a baking peel or a flat baking sheet with flour. Remove the plastic bag and gently invert the dough onto the peel. (The dough may deflate somewhat.) Remove the basket and towel. With a single-edge razor blade or a sharp serrated knife, make a few slashes in the surface of the dough.
A few slashes with a razor blade allow the bread to rise evenly. Uncut bread will give you a haphazardly ripped crust.
With a quick jerk, slide the dough from the peel to the baking stone. Wearing long oven mitts and standing as far away from the oven as you can, immediately pour the boiling water into the metal pan in the bottom of the oven. Caution: This will cause an instant burst of steam. Close the oven door immediately and don’t open it for at least 10 minutes or the steam will escape.
Bake the bread until it is well browned and sounds hollow when thumped on the bottom, 35 to 40 minutes. Cool completely on a rack before serving.
To keep the crust crisp longer, store bread in a paper bag or on the kitchen counter.