In a small bowl, sprinkle yeast over warm water along with sugar.
Stir briefly to mix and let stand until bubbly, 5-10 minutes.
In a large bowl, put flours and salt.
Add cold water to yeast mixture, mix well, and add to the flour.
Knead in the bowl until combined, adding more water if necessary, then transfer to lightly floured (or oiled) counter or other flat kneading surface.
Continue to knead until dough is smooth, about 15-25 minutes by hand. Form into a ball.
Return dough to bowl (make sure your bowl is big enough to accommodate the dough doubling or tripling in volume).
Cover with plastic wrap or clean kitchen towel and set in a warm place. About 75-85 degrees F is best. Your oven, turned off but with the oven light on, is a great place to let dough rise. If you want to hurry the process along, you can also turn your oven on to the lowest setting (mine goes down to 170 degrees F — oh how i wish for a Miele!) for 2 minutes then turn it off — do this before you mix the dough, and leave the oven door closed.
Let the dough rise for about an hour, until it’s about 1 1/2 times its original volume.
Transfer it back to the work surface and shape it roughly into a rectangle approx. 14 inch long.
Fold 1/3 down and fold the lower third up (kinda like a brochure).
Press again to a 14-inch long rectangle and repeat folding.
Return to bowl and let rise again, covered, about 1 1/2 hours or until 3 times its original volume.