Yeast Starter: In a small bowl, add ½ cup lukewarm water, active dry yeast and 1 tablespoon sugar. Let rise and foam. Set aside.

Tsoureki: In a saucepan, add milk and butter. Set to low heat until butter has melted and milk is warm. Using an electric mixer on low speed, add the warmed milk and butter. Next add the eggs and sugar. Add the orange juice, zest, almond extract, vanilla extract, mastic and mahlep. Add a pinch of salt and then add in the foamy yeast starter. Turn mixer off. Now with your hands or using a dough hook on low speed, add in the flour one cup at a time. Knead for 5 minutes. Now set aside dough to rise covered with plastic wrap or towel in a warm place. Let rise for 3 hours or until dough has doubled. 

Using the 1/4 cup olive oil, grease the dough and your hands to make 6 equal dough balls. Now roll out each ball into a 9 in strand equal in size. Combine the strands at the top of each Tsoureki and start braiding it. Tuck the bottom underneath the Tsoureki and now allow to rise for another 30 minutes. Brush the egg wash and sprinkle almond slivers. Bake at 375 degree F or until golden brown about 30 minutes depending on your oven. Serve warm or let cool and cover in plastic wrap until the day you would like to serve it