Melt the butter and add it to the milk - this should then be lukewarm - add the warm water and sugar, stir until the sugar has dissolved. Check for temperature; blood temp, no hotter. Add the yeast, and leave to activate and become foamy.Note: if the water/milk/butter is hotter than blood temp. you can kill the yeast.
Once foamy, whisk the eggs and add them to the yeast mix, stir thoroughly combine.
In the mixer's bowl, combine the flour, zest, and spices, mixing thoroughly. Use the dough hook, and add the yeast mix. On a low speed, mix until it's all combined and forms a loose ball.
Continue to 'knead' the dough in the mixer bowl for a further 10mins, on medium speed. The dough should look silken and smooth, but have a slightly sticky feel when touched. Try not to add additional flour, as the dough relies on moistness for it's soft texture.
Place the tea towel over the bowl, and leave to prove until it has doubled in size - anywhere between 1-3hours temperature depending.
Knock-back the dough to deflate it, and remove the dough to a floured surface. If possible, weigh the dough and divide evenly into 3 equal portions, and roll out to make long and thick strands.
Line the three strands next to each other, and pinch the top together. Once secured, plait the three strands, being careful to pinch the end as you did the top. Tuck the pinched ends underneath, so it looks pretty.
Place a sheet of parchment paper on your baking tray, and carefully lift the shaped tsoureki onto the tray. Cover with a tea towel, and leave to prove again for approx. 30mins, or until it has almost doubled in size again.
Whisk together the 'decoration' egg and water, and brush the top of the loaf. Once that's done, scatter the slivered almonds across the top - be careful not to deflate the loaf!
Bake at 160˚C for about 25-35 mins. or until golden brown.
The syrup is optional, but does add an element of luxury. To create the syrup, dissolve the sugar in the water over medium heat, add the cinnamon quill, and boil for approx. 2mins. When the tsoureki is ready, remove it from the oven, and immediately brush over the syrup, allowing it to soak in.
Leave the loaf to cool to room temp. before eating, and enjoy with a hot drink (it's great at breakfast!).
Keep your tsoureki in a tied plastic bag, and it should last 5-7 days at room temp.
Note: Traditional tsoureki uses mastica and mahlep for flavouring, but sadly these are rarely available outside of Greece. The spice combination used is a rough approximation of the combination.