Using the heel of one hand to lean into the dough and the other to rotate it, knead until a very smooth dough forms, another 9 to 10 minutes. If you get tired, rest for a second, throw some dishes in the sink, then keep kneading for another minute or two. You should be able to hold the ball of dough and not have it stick to your hands, but it also shouldn’t feel too dry or tear across the top; you’re looking for smooth!

Add the remaining 1/2 tablespoon oil to a large metal bowl and use a pastry brush or your hands to coat the bowl with the oil. Add the dough, flip it to coat lightly in oil, seal with plastic wrap, then cover with a clean kitchen towel and let the dough sit in a warm place until at least doubled but ideally tripled in size, 1 hour 30 minutes to 2 hours for double and 3 hours for triple (tripling makes the dough lighter, so if you have time, I recommend it). Uncover the dough and punch it down with your fist, then transfer the dough from the bowl to the work surface.

Cut the dough into 2 equal-sized pieces (or 3 for smaller challahs), then cut each larger piece into 3 smaller pieces.

Line a large baking sheet with parchment paper. Press down on each piece of dough to release any air bubbles, roll each into a 12-inch log (or a 10-inch log if you are making 3 challahs), then roll out and taper the ends of each log to form 16-inch logs (or 12-inch logs for 3 challahs). Let the logs rest for 5 minutes to allow the gluten to relax. Pinch the top ends together, pressing down the pinched top with a chef’s knife to secure the dough. Braid the logs, forming a challah that is slightly tapered on both ends and plump and generous as it reaches the center. Tuck the ends under and arrange the loaves on the parchment-lined sheet, leaving at least 4 inches between loaves. Let rise, covered, in a warm, draft-free place, until the challahs puff and nearly double in volume, 1 hour.

During the last 15 minutes of rising, set a rack in the center of the oven and preheat the oven to 350°F.

Beat the remaining egg in a small bowl, then brush the challahs thoroughly all over with the egg, making sure not to neglect the sides of the challah all the way down to the parchment (so that the sesame seeds will coat the whole challah). Coat each challah with 1/4 cup of the sesame seeds (or about 2 1/2 tablespoons if you’re making 3 challahs). Sprinkle with the flaky sea salt and bake until the challahs are golden and fluffy and the undersides are a lovely golden brown color and sound hollow when tapped, 34 to 36 minutes for 2 large loaves, 28 to 30 minutes for 3 medium loaves.* Cool and slice.