FOR THE SPONGE, whisk yeast into water to disperse and stir into flour until smooth. Cover and let rest for 30 minutes.
FOR THE DOUGH, combine flours and form a well, placing salt and sponge inside. Mix to form dough. Knead 8-10 minutes until smooth and elastic. Place in a lightly oiled bowl, cover and let rise until doubled. Saute onion in oil until lightly browned. Blot off all excess oil. Deflate dough by pressing out with your hands. Gently knead in onions and allow to rest for 10 minutes. Divide dough into 3 pieces, place in greased casseroles, and make 3 slashes in the top of each loaf. Cover with oiled plastic wrap and allow to rise until doubled. Bake at 425F for 20 minutes. Makes 3 Loaves