Take the outer two ropes and cross them over each other to switch places, crossing the rope from the right under the rope from the left. Take the farthest rope on the right and cross it over to be in the middle (with 3 ropes on the left of it and 2 ropes on the right). Then, take the second rope from the left and cross it all the way to the far right. Now, take the farthest rope to the left and move it to the middle (with 2 ropes on the left of it and 3 ropes on the right). Take the second rope from the right and cross it all the way to the far left. Repeat this process until there are no more ropes to braid, then pinch the ends and tuck them under the end of the challah. Using your hands, carefully transfer the challah to a parchment-lined half-sheet pan, placing it on a diagonal. Beat the remaining egg and brush liberally on the challah. Let the challah rise again, uncovered until doubled in volume, about 1 hour.