1. Mix the 1 cup flour, 1 cup water and yeast together in a large bowl. Let the mixture sit for about 45 minutes.
2. Add the dough ingredients to the starter and mix and knead together — by hand, mixer or bread machine — until a smooth, supple dough is formed. Add a few tablespoons more of flour if you find your dough is very sticky still after a few minutes of kneading. I used a couple more tablespoons and kneaded the dough in my mixer for about 10-15 minutes.
3.Place the dough in a greased bowl, turning it over once to coat it lightly with oil. Cover it and let it rise for 1 1/2 hours, or until it's not quite doubled in size.
4.Turn the dough out onto a lightly floured surface and fold it over once or twice, to expel the carbon dioxide. Divide the dough into four pieces, and roll each into a snake about 18" long.
5. On the lightly greased or parchment-lined baking sheet, braid a four-strand braid (see instructions in our blog,) or fashion a simpler three-strand braid.
6.In a small bowl, make the glaze by mixing together the reserved egg white, sugar, and water. Brush the loaf, reserving some for a second application.
7.Cover the loaf with lightly greased plastic wrap and allow it to rise for 45 minutes to 1 hour, or until it's almost doubled in size. Towards the end of the rising time, preheat the oven to 375°F.
8. Brush the loaf with the remaining glaze (this will give the finished loaf a beautiful, shiny crust, as well as provide "glue" for the seeds), sprinkle with poppy seeds, if desired, and bake for 35 to 40 minutes, or until the challah is golden brown, slightly firm to the touch, and the internal temperature is 195°F.
9.Remove the bread from the oven, and cool completely before slicing. Store completely cool bread, well wrapped, at room temperature for several days; freeze for longer storage.