Stir together the flour, sugar and yeast in a large bowl. Make a well in the centre.
Whisk together 2 whole eggs and 1 egg yolk along with the oil, orange zest and honey in a bowl. Measure 200ml lukewarm water (if it’s too hot it will kill the yeast) into a jug.
Put the egg mixture into the well, then start to mix and pull in the flour from the sides of the bowl. Begin to pour in the water to form a thick and slightly sticky dough, with everything evenly combined – you may not need it all. Turn the dough out on to a lightly floured surface, and knead for about 10 minutes. The dough should be smooth and silky.
Oil a large bowl and place the dough inside. Cover with clingfilm and leave to rise in a warm place for about an hour, or until doubled in size. Separate the dough into 6 equal pieces, and roll into strands approximately 3cm thick and 40cm long. Gather the ropes together and squeeze them at the top, lining all the strands lengthways next to each other.
Begin plaiting by lifting the outside right strand and placing it over the two beside it, then under the middle rope, and over the remaining two. Lay the rope you have just moved parallel next to the others. Repeat this pattern to the other ropes, always starting from the right and finishing at the left. Try to keep the braid as tight as possible. You can gently pick up the loaf and straighten it up if it starts to veer towards the left. Gently cup both ends of the braid and push them towards each other under the loaf, so it forms a more loaf-like shape.
Line a baking sheet with greaseproof paper and slide the loaf on to it, then cover with a clean tea towel and allow to rise again for around an hour, until it has puffed up. While the loaf is rising, preheat the oven to 190°C/gas mark 5. Brush the loaf all over with the egg white and bake in the oven for 25-30 minutes.
Meanwhile, make the orange blossom syrup. Add the sugar to a saucepan with the orange juice and 50ml water, then bring to the boil. Add the orange blossom, then simmer until syrupy. Stir through the pistachios.
Carefully brush on the pistachio syrup 5 minutes before the bread is done. It should be golden with a nice sticky glaze. Cool slightly before slicing to serve.