Recipe yields 1 loaf; can be doubled so long as the baker’s percentages stay the same.
In a medium bowl, mix all starter ingredients. The starter will be thick and stiff, but work at it until all ingredients are incorporated; you can let the mixture sit for about a half hour and then mix again to thoroughly combine if necessary. Cover the bowl and let it sit at room temperature overnight.
In a large mixing bowl, combine the water, sugar, oil, three eggs, and salt; whisk until well combined and the sugar and salt are dissolved. Mix in the flour using a wooden spoon or your hands. At this point, the dough will be shaggy, stiff, and dry.
Flour your work surface and turn out the dough. Add the prepared starter (I try to make a large divot in the middle of the dough to corral it) and knead the dough until it is smooth and satiny (about 10 minutes); add a bit more flour to help with sticking, or add a tablespoon or two of water if the dough is so stiff that it’s hard to knead. Place the worked dough in a greased or oiled large mixing bowl; cover with plastic wrap and let it sit at room temperature for two hours.
Line a baking sheet with Gefen Parchment Paper and set aside for now.
Divide the dough into four equal portions. (If you prefer a three-braid challah, divide the dough into three equal portions.)
Cover the dough pieces with a towel and let them rest for 15 minutes.
Roll each portion into a long, thin log about the length of the baking sheet. If the dough keeps wanting to shrink back, cover the dough again and let it continue to rest for another 15 minutes.
Place the log strands side by side on the baking sheet and pinch the four top ends together. To braid, grab the log on the far right; pass it over the next strand, under the third strand, and then over the fourth; the log strand that started out on the far right is now on the far-left side. Repeat this braiding, always starting from the far-right side, until the entire loaf has been braided. Pinch the ends at the bottom and then tuck both ends underneath the loaf to hide the pinches.
Cover the braided loaf with plastic wrap (don’t cover it tightly, though) and let the dough rise until well bulked up—at least doubled or even tripled in size (five to seven hours).
Preheat the oven to 350 degrees Fahrenheit.
While the oven is preheating, beat the last egg in a small bowl until very well mixed. Brush the top of the braid with the beaten egg and then sprinkle the seeds on the top if using. Bake the challah for 35 to 40 minutes in the upper third of the oven.
If the bread seems to be browning too quickly, place a tent of aluminum foil over the top to finish baking.
Set the challah bread on a wire rack until it is completely cool.
Recipe excerpted with permission from The Homestead Sourdough Cookbook. (Ten Peaks Press). Buy the book on Amazon.