Mix together the warm water, 2tbsp sugar and yeast and leave to bubble up while you mix the rest.
In a large bowl, add the flour, salt, sugar and mahleb, as well as mastic and/or orange zest if using. Mix them together so well combined.
Add the melted butter, eggs, milk and the bubbled-up yeast mixture to the bowl and mix well. You will probably need to finish it off by hand to get the last of the dry ingredients incorporated.
Tip the dough onto a clean surface and knead for around 5 minutes to stretch the dough. It should feel pretty dry - if it's a little sticky add a little more flour.
Transfer the dough to a bowl that has been lightly brushed with oil and cover. Set aside in a slightly warm place for a good 2 - 3 hours until it has doubled in size.
Once it has doubled, knock back the dough (gently deflate it with your fingers), then divide into three pieces. Stretch out and roll each piece to give you three long 'sausages'. Put the pieces on a baking sheet or piece of parchment to make it easier to transfer. Join the three pieces together at one end then braid them together. Join the other end. Make sure you don't braid too tightly as it still has to rise again.
Set aside for around another hour to rise again - roughly double or a little less. When near the end, preheat oven to 350F/180C.
Mix together the egg and milk, beating lightly, then use this to glaze the loaf. Take care not to deflate it as you brush it. Sprinkle over the sliced almonds and then bake for approx 45 minutes, covering with a foil 'tent' after around 20 minutes to stop it browning too much. The loaf should be a golden brown. Transfer to a cooling rack and let it cool to gently warm before slicing.