In the bowl of a standing mixer with a dough hook, put flour, sugar and yeast (or in a large mixing bowl if kneading by hand) and mix on a low speed for about a minute.
Add egg and oil, and mix for one minute.
Slowly add water and mix to form the dough .*Because each flour is different, stored differently, and has different absorption capacities, the amount of liquid will almost always change. Even if you always use the same flour, it may be that on different days, the flour will absorb more liquid than before. So, start with half a cup of water and gradually add enough until the dough is soft but not sticky.
Add the salt and knead for 12-15 minutes (It helps the gluten form and the dough to be elastic with better structure). Scrap the sides of the bowl occasionally with a rubber spatula to ensure all ingredients are incorporated.
Transfer dough to a lightly floured surface. Using floured hands, knead the dough for 3-5 minutes. You should have a smooth ball of dough.
Grease the dough with a little bit of oil, cover with a kitchen towel and let the dough rise until doubled (place the bowl somewhere warm).
Separate the dough into 3 equal pieces.
Roll each piece of dough into a long rope.
Gather the ropes and squeeze them together at the very top. Braid the ropes together like braiding hair and squeeze the ends together when complete.
Line a baking sheet with parchment paper and place the Challah on it.
Place the pan somewhere warm, cover with a towel, and let it rise until puffed, about an hour.
Heat the oven to 400°F/200°C. When ready to bake, whisk the egg with a tablespoon of water and brush it all over the Challah.
Sprinkle on sesame seeds.
Slide the Challah on its baking sheet into the oven and bake for 25 to 30 minutes. The Challah is done when it is a rich golden brown. For precise assessment of their doneness, use an instant-read thermometer to measure their internal temperature- it should be around 190°F/90°C when done. Remove the pan from the oven.
Cool challah in the pan for at least 10 minutes before transferring to a cooling rack until ready to slice and serve.