In a large bowl or the bowl of a stand mixer fitted with a dough hook, combine the warm water, sugar and yeast. Let it rest for about 5 minutes, until it becomes foamy.
Add flour, oil, eggs, egg yolk, and salt to the bowl. Mix on medium-low speed for 8-10 min until the dough is becoming sticky and elastic. The dough will cling to the bottom of the bowl while pulling away from the sides and slap against the sides of the bowl. If necessary, after 6 minutes, add flour one tablespoon at a time.
Shape the dough into a ball, place in a lightly oiled bowl, turn to grease the ball, cover the bowl, and allow the dough to rise in a warm, draft-free location (75°) until it has doubled in size, about 1 to 2 hours.
Transfer the dough to a floured surface after it has risen. Divide the dough into 6 equal portions and roll each into a 12-inch long rope. Lay all 6 ropes parallel to one another, pinch the ends together and fan them out slightly.
To braid the dough start with the outer right rope and cross that rope over two strands toward the center. Then taking the second from the left strand to the right over all, this one must be placed on the outer right. Next, take the outer left strand and overpass two strands to the center. Then take the second rope from the right and move it to the outer left. Continue this pattern until the whole loaf is braided. Slightly pinch the ends and tuck them underneath. Place the plaited loaf on a parchment paper-lined baking sheet. Loosely cover and let it rise another 30 minutes to an hour, or until almost doubled in size.
Towards the end of the final rising time, preheat your oven to 350°F. Beat the egg white in a small bowl till frothy. Brush the egg wash on the loaf. Sprinkle with seeds if desired.
Bake 20 to 25 minutes, or until the loaf is deep golden brown. You can also check for doneness with an instant-read thermometer—the internal temperature should be between 195 and 205 degrees Fahrenheit. Once baked, transfer the loaf to a wire rack to cool completely. Challah is best fresh on the day it is baked, but it may be wrapped tightly in plastic and kept at room temperature for a day or two, or stored in an airtight container for a few days in the fridge.