Adjaruli khachapuri, the famous Georgian cheese bread with an egg on top, homemade using a sourdough starter.
Combine the sourdough starter with the flour and water in a plastic bowl. Stir until all the flour has been incorporated. Cover with cling film and leave out at room temperature for about 8 hours or overnight.
Melt the butter and let cool in a small bowl, as you don't want to add it hot to the dough. Lightly warm the milk to have it lukewarm and crack the egg in it (the egg should be at room temperature). Whisk the egg and milk mixture.
In a larger bowl, combine the 150 g of flour with the prefermented levain from the day before and add the milk and egg mixture. Mix these ingredients with a wooden spoon or rubber spatula. When the dough is starting to come together, add the salt and melted butter and keep mixing until homogeneous. The dough should be soft but not sticky. Cover the bowl with cling film and let rest for 2 hours.
Drain the mozzarella and coarsely grate it on a grater. Place grated mozzarella on a colander to let excess liquid drain. Crumble the feta and mix together with the mozzarella.
After 2 hours have passed, transfer the dough to a floured surface and divide it into 2 balls. Roll them out with a rolling pin trying to get a long oval shape on both. Transfer rolled out dough to an oven tray lined with baking paper.
Divide filling between two pieces of dough, leaving about 1-2 cm from the edges. Fold the side edges over the filling on both sides, then press ends together and give a slight twist to securely close them. This will prevent the dough from snapping open as it bakes, losing the boat shape and letting the filling pour out.
Bake in the preheated oven at 250°C (480°F) for 12 minutes. During this time, the cheese should melt completely and the dough should get a golden tone.
After 12 minutes, take khachapuri out of the oven and crack an egg in the middle of each filling. Divide room temperature butter between the 2 fillings, then return the breads to the oven for 3 more minutes. The egg white should be cooked enough to lose its translucent appearance, but the yolk should still be runny when the bread is served.