1. Mix the yeast into the water, add a spoonful of sugar and let it stand for a few minutes to froth up. Place the flour, salt and the remaining sugar in a large bowl or the bowl of a standing mixer. Add the eggs, the egg yolk and the oil into a well in the middle of the flour. Mix briefly, pushing the flour off the sides into the liquid in the middle.
2. Pour the yeast mixture over the dough. Mix into a rough dough, then knead by hand on a floured surface for about 10 minutes or in the standing mixer with a dough hook attachment, for at least 5 minutes. You might want to add more flour if the dough seems very sticky. You’re aiming at smooth, elastic dough that stops sticking to your hands or bounces off the sides of the bowl. Cover and leave it in a warm place for 1 – 1 ½ hour until doubled in size.
3. Turn the dough out onto a floured surface and divide into three equal pieces (this is the easy braid, just like a plait, you can have a go at six braids if you like a challenge). Roll each piece out into a long rope, about 40cm long. If they spring back, let them rest for 5 minutes and try again.
4. Make the braid by bunching the three ropes together at one end, place each outside rope on top of the middle one, just like plaiting hair. Pinch together the other end and tuck both underneath the loaf to shape neat ends.
5. Place the loaf carefully on a baking sheet lined with parchment and sprinkled with a little flour, dust the loaf with more flour, sparingly, cover loosely with a clean tea towel and leave to prove and puff up considerably for about 30-40 minutes.
6. Preheat the oven to 180C/350F/gas 4. Beat the reserved egg white with a couple of tablespoons of water and brush all over the loaf, making sure it goes into all the cracks. Sprinkle with poppy seeds, if using.
7. Bake on the middle rack for 35 – 40 minutes, turning the loaf around halfway through. Cool on a wire rack.