Combine all of the sponge ingredients, mixing just till a cohesive dough forms. Allow it to rest, covered, for 12 to 16 hours at room temperature. When the sponge is ready, it will be filled with large holes and bubbles.

Add the water to the sponge, and mix till smooth. Add the flour, yeast and salt, and knead the dough till it’s fairly smooth but not necessarily elastic, about 3 minutes by machine, or 5 minutes by hand. (The gluten will continue to develop as the dough rises, so you don’t want to develop it fully during the kneading process.)

Place the dough in a lightly greased bowl, cover, and allow the dough to rise at room temperature for 1 ½ hours. To help develop the gluten and distribute the yeast’s food, turn the dough twice during the rising time: gently fold all four sides into the middle, and turn the dough over.

Divide dough in thirds, and roll each third into a 20-inch-long rope. Braid ropes. Set the braid on a lightly greased or parchment-lined baking sheet, cover with greased plastic wrap and let rise 1 to 1 ½ hours, until just puffy.

Preheat oven to 425F. Gently brush the braid with the beaten egg white mixture and sprinkle generously with sesame seeds. Bake for 25 to 35 minutes. Remove from the oven and cool on a rack.