In a large pan or skillet, saute the mushrooms and then the onions, adding garlic in the last 5 minutes of sauteeing. Then cook the lamb in the pan (breaking it apart into small chunks with a spatula), adding the spices, pine nuts, mint, and black garlic. Once lamb is cooked through, combine it with the mushrooms and set aside.
Preheat oven to 400 degrees.
In a bowl, mix together the ingredients for yogurt wash. On a clean dry surface, place one sheet of phyllo dough. With a silicone basting brush, lightly brush the phyllo sheet with this yogurt brush. Then place a new phyllo sheet on top of this and brush a bit more yogurt wash on top. Continue this process until you have 4 sheets.
Now, add a fourth of the lamb mixture in a thin band along the long side. Now roll it up from the filling side and place aside. Repeat this process for four more pastry rolls.
When completed, place one pastry roll into the middle of a cast iron pan and roll it into a spiral. Add each remaining pastry end-to-end until you’ve used all the rolls. Brush some melted butter on top and sprinkle with sesame seeds. Bake in oven for 35-40 minutes until golden on top.
Mix together ingredients for yogurt sauce. Cut the borek like pizza slices and serve topped with yogurt sauce.