In a small bowl, add the sugar and yeast, and give a little stir. Pour the lukewarm water over the yeast, and let sit for 10 minutes, or until the yeast blooms.
In a large mixing bowl, combine the bloomed yeast mixture, honey, salt, egg, and 1¼ cup water. Mix it up with a mixing spoon until it's combined.
Add a cup of flour and stir vigorously until combined. Do this again two more times.
Remove the mixing spoon and add the rest of the flour. With your hands, get as much of the dough off of the mixing spoon as possible, and begin combining everything in the bowl until the dough has incorporated all of the flour.
Drop the dough onto your worktop and knead the dough for about 5 minutes, adding flour if it's sticking to your hands and not coming off. It's okay if it's a slightly sticky, but you want to be able to knead it properly without it remaining on your hands. Form the dough into a neat ball.
With 2 tsp oil, grease a large bowl and place the dough ball in. Cover with a kitchen towel. Put the boiling water into a medium bowl and put on the floor of your oven, turned off. Place the covered bowl with the dough in it onto a rack in the oven and leave for 2 hours to proof.
Remove the dough from the oven (as well as the bowl of water). It should have doubled in size. Poke or pound down into the dough all over so it deflates a bit. Take it out of the bowl and put it onto your worktop. Using a bench scraper or knife (or just by tearing it), divide the dough into 3 equal balls. Take one ball and place it in front of you and set aside the other two, covering them with a kitchen towel.
Divide the dough again into 3 equal chunks (or more if you're using more strands to braid with). I like to use a kitchen scale to ensure they're EXACTLY equal, but this is optional. Roll your dough chunks into a long "snakes" until they're about an inch in diameter.
Arrange the dough "snakes" next to one another and pinch them together at the top. I like to push down and kind of smear them a little bit into the worktop so that they stay put while I braid. Proceed to braid your challah, pinching again at the bottom and then tucking the end underneath the challah.
Flip your challah around so the top is now the bottom and tease the strands away from one another and braid them as well, pinching them at the end and tucking the end under the challah like before. Cover your braided challah with a kitchen towel and set aside for 30 minutes to rise. While the first challah is rising, preheat the oven to 350°F.
Repeat the braiding process with the other two balls of challah dough, making sure to cover them and let them each rest after braiding.
Once your challah has rested, place it onto a baking sheet lined with parchment paper (or an oiled bread tin) and brush the top of it with egg yolk. Finally sprinkle with sesame seeds to give a nutty, distinctly Jewish flavor. Of course, the sesame seeds are optional, and can be replaced with just about any topping. Some examples that come to mind include everything bagel seasoning, za'atar, Maldon salt, streusel, sprinkles... the sky's the limit!
Bake at 350°F for about 15 minutes and then remove from the oven and egg yolk wash where the challah expanded and therefore doesn't have any egg wash on it. Return the challah to the oven for 20-25 minutes, or until golden.
Remove from the oven and make sure that the challah is cooked through. You'll know it is when you gently try to tease braids apart (on the end or somewhere else inconspicuous)- there should be no evidence of doughiness, and the braids want to stay intact, almost like there are strands of gluten holding them together. Remove the challah from the tray and eat warm or place on a challah plate, serving plate, or cooling rack until cool. Slicing will be easier once the challah is cooled.