Prepare the yeast. In a bowl mash the yeast, add the tepid water, 1 teaspoon of sugar and 150g flour. Stir well until you get a sort of loose paste. Cover the bowl and let it stand in a dark, warm place for 20 minutes until it becomes frothy. (If possible put the bowl on the stove or close to a radiator and try not to move it at all). Place the butter in a glass bowl. Place the bowl over a saucepan with barely simmering water. When the butter melts add the sugar and whisk until dissolved. Add the eggs and whisk briskly. Make sure all the ingredients are at room temperature and keep the heat very low. Crush the mahlab, mastic and cardamom pods in a pestle and mortar and mix them with 1 tablespoon of the sugar. Sieve the flour over a large bowl. Add the yeast and the butter-sugar-eggs mixture. Add the spices and work the dough with your hands until it holds together and leaves the sides of the bowl clean, Sieve some extra flour if needed.
Cover the bowl and let the dough rise in a warm place, for approximately 1h, until it doubles in volume.
Divide it into 3 equal pieces. Roll each into a baguette (long and narrow loaf of bread) and plait the three together. Line a flat baking tray with greaseproof paper. Place the tsoureki loaf, cover it and wait until it doubles in volume. Preheat the oven to 160-170ºC. Brush the tops of the tsoureki loaf with some beaten egg and water and bake it for 45 minutes until golden. If you brush it with butter the moment you take it out of the oven, your tsoureki will become glossy.