Add the water and the sourdough starter to a bowl and with a whisk bread down the starter

Add the egg, the yolk, and the sugar and combine everything

Sift the flour and salt together and add them to the dry ingredients

Combine everything until you don't see dry flour particles and let it rest 30-60 mins

Start kneading, if using a stand mixer you can use the paddle attachment

When the dough starts developing the gluten add the oil and keep kneading until it's completely absorbed (see notes)

Knead 4-5 minutes and let it rest another 5-7 minutes. Continue doing thes until it's ready (Let the dough rest as much as necessary).

When the dough passes the windowpane test and it's smooth and elastic, put it in a lightly oiled container and let it ferment until it doubles or almost doubles and feels puffy (at 68-74F it takes tops 8h, typically overnight)

Degas the dough and let it rest a few minutes. Then divide it in as many strands as you want for your braid, shape them into dough balls and let them rest (covered) for 10 minutes

Take one piece of dough and with the heel of your hand flatten it. Use a rolling pin to roll the dough into a 7 x 4 inches oval. Starting from the longe side roll the dough into itself and form a log. Pinch the seam closed and gently roll from the center outwards lengthening the log

Repeat with all the dough pieces, if it's difficult to stretch the log, let it rest a few minutes

Braid the challah and place it on a piece of parchemnt paper on the tray you'll use to bake it. Cover it with plastic wrap and let it rise until it feels puffy (at 68-74F it takes about 3-4h)

Preheat the oven for 20 minutes at 350 F. When the challah is ready, brush it with the egg white and sprinkle poppy or sesame seeds. Bake it for 35-40 minutes or until it has a nice golden brown color

Let the challah cool down to room temperature before slicing it