The flour/liquid ratio is important in this recipe: Weigh your flour; or measure it by gently spooning it into a cup, then sweeping off any excess.
To make the dough, combine all of the dough ingredients, in the order written, in a very large bowl (at least 6 quarts). Stir to make a tacky dough. (If your dough is extremely sticky, add flour by the tablespoon until it is tacky.) You can do this in a stand mixer and then transfer to a larger bowl; or simply use a big bowl and spoon or dough whisk.
Cover the bowl, and let the dough rise for 2 hours at cool room temperature.
Refrigerate the dough for at least several hours, to chill it and make it easier to work with. It can be refrigerated for up to 4 days before using.
When you're ready to make challah, remove the dough from the refrigerator, and scoop out about one-third of the dough (about 550g). Transfer it to a floured work surface; a silicone mat works well here.
Divide the dough into 3 pieces (about 183g each) and shape each piece into a log about 24" long, sprinkling more flour as necessary to prevent sticking.
Braid the logs, pinching the strands together at each end.
Lightly grease an 8" x 2" round cake pan. It's important that the pan is at least 2" deep. If it's not, use a 9" round pan. Coil the braid into the pan.
Cover the pan, and allow the braid to rise for about 90 minutes, till it's quite puffy. Towards the end of the rising time, preheat the oven to 350°F.
Uncover the challah, and brush it with the egg/water mixture. Sprinkle with seeds.
Bake the challah for 30 to 35 minutes, till it's golden brown and an instant-read thermometer inserted into the center registers at least 190°F.
Remove from the oven, and carefully turn out of the pan onto a rack to cool.
