Feta filling: Heat olive oil in a pan over medium heat. Add onion and garlic. Sauté until translucent. Add nutmeg and cumin and stir. Remove the pan from the heat and let cool completely.

In a food processor, pulse the spinach, parsley, rodopa cheese, egg, and the onion mixture until it becomes a chunky paste. Transfer to a bowl and set aside.

Melted Kashkaval filling: Add parsley, chives, egg, and Kashkaval cheese to the food processor and pulse until it becomes a chunky paste. Transfer to a bowl and set aside.

Preheat your oven to 375°F. Line 2 baking sheets with parchment paper.

Boeregs: Unroll the filo dough and cut crosswise into 3½-inch wide strips. Cover the dough with a damp towel while filling the boeregs.

Lay a strip of dough lengthwise on a cutting board. Brush a thin layer of butter down the length of the dough. Scoop ½ tablespoon of filling at the top center of the strip. Fold the top left corner of the dough over the filling to make a triangle. Brush more butter on the top, then continue folding and brushing until you reach the end of the dough strip. Transfer to a baking sheet and cover with a kitchen towel. Repeat with the remaining dough strips and filling.

In a small bowl, beat the egg yolks with water.

Brush the egg wash over the boeregs, then sprinkle with the sesame seeds.

Bake boeregs for 10-12 minutes or until golden, flaky, and crispy.