Crumble the feta cheese into a mixing bowl, ensuring there are no large chunks.

In a separate bowl, whisk together the three eggs and sour cream until smooth and well-combined.

Add the crumbled feta to the egg and sour cream mixture. Pour in all but 2 tablespoons of the reserved milk, and mix everything together until thoroughly combined.

Gently melt the unsalted butter in a microwave or a small saucepan over low heat.

Preheat your oven to 360°F (180°C). Brush a baking dish with a generous layer of the melted butter.

Take two sheets of phyllo pastry and place them in the prepared baking dish, brushing melted butter between the layers.

Bunch up four additional phyllo sheets, then place them on top of the first layer. Brush with melted butter, then add half of the cheese mixture evenly over the phyllo.

Add another four bunched phyllo sheets on top of the filling, brush with butter, and add the remaining cheese mixture over this layer.

Top the borek with two more sheets of phyllo, brushing each one with butter.

In a small bowl, mix the reserved egg yolk with the remaining milk and brush this mixture over the top layer of phyllo.

Sprinkle nigella seeds on top, if desired, then place the baking dish in the preheated oven. Bake for 50–60 minutes.

Once baked, remove the Hearty Borek from the oven and let it cool for 5–10 minutes before slicing.