Mix yeast with 1 teaspoon of the sugar and 250ml water in a small bowl. Let stand for 10 minutes until it becomes frothy.
In a large bowl combine the yeast mixture with the remaining sugar, beaten eggs, salt, oil and flour; work into a smooth dough by hand or by using the dough hook in a freestanding mixer.
Take the dough out of the bowl and lightly oil the bowl. Place the dough back into the bowl. Turn the dough over to coat evenly with oil. Cover with a clean tea towel and let rise for 2 hours in a warm place.
Take the dough out of the bowl and punch it down with your fists to remove any air pockets. Transfer it to a floured work surface and cut into two equal portions.
Divide each dough portion into three equal parts. Shape each part into a long rope of equal length and thickness. Loosely plait the three ropes together. Pinch the ends together to seal. Place the plait on a baking sheet lined with parchment paper. Proceed the same way with the rest of the dough. Place the second plait at a generous distance from the first, as the dough will expand considerably. Cover with a clean tea towel and let rise for 1 hour.
Preheat the oven to 180 C / Gas 4. Mix the egg yolk with 1 tablespoon water. Brush the challah with this mix right before they go into the oven and sprinkle with sesame seeds if desired.
Bake on the middle rack of the preheated oven for 30 minutes, or until golden brown. Let cool on a wire rack.