Combine yeast with 100g of the flour and add water. Let stand for 15 to 20 minutes, until the mixture foams.
Add 2 eggs, sugar, salt and oil to the yeast mixture. Add the rest of the flour, stirring with a wooden spoon. Form a ball from the dough, lay on a floured work surface and knead for about 10 minutes, until the dough is smooth and no longer sticky.
Place dough into an oiled bowl, cover with cling film and let rise for 2 to 2 1/2 hours in a warm place. The time required for the dough to rise will depend on the room's temperature. Dough should double in size.
Once the dough is ready, divide in half. To plait challah, cut each portion in four even parts (for a plait of 4 strands) or in thirds for a regular plait. Shape each piece into a rope about 30cm long.
Grease a baking tray or line with parchment. Plait each challah and lay on tray. Cover with oiled cling film. Let rise for 1 to 1 1/2 hours, till loaves have tripled in size.
Brush each challah with lightly beaten egg white and sprinkle with poppy seeds.
Bake in a preheated 180 C / Gas 4 oven for 25 to 35 minutes until golden brown. Let cool on wire racks before slicing and serving.