In the bowl of your Bosch mixer fitted with the dough hook mix yeast, 1 Tablespoon of sugar and water.
Let sit for 5 minutes.
And in the rest of the challah ingredients, except for the flour.
Add 7 cups of flour (in Israel a 1 kilo bag) and turn your mixer on. Start on low and gradually increase to full speed.
If dough is very sticky add flour by the ½ cup.
If dough is ok, or even a little sticky, keep mixing for about 5 minutes.
Turn off mixer and let dough rest for 3 minutes.
After 3 minutes of rest, touch the dough with your fingers. If it is sticky, add flour by the Tablespoon.
If dough is not sticky, turn on mixer once more for another 3-4 minutes. The dough should "clean" the sides of the bowl.
If the dough is still sticking to the sides of the bowl, add flour by the Tablespoon and continue the mixing, resting until the bowl is clean. This may take some time, be patient.
When the dough is the right consistency after letting it rest 2-3 minutes remove it from the mixing bowl on to the counter and form a large ball.