Prepare the spinach: Heat a large skillet over medium heat and add the chopped spinach. Cook for 2–3 minutes, stirring occasionally, until wilted. Remove from the heat and allow it to cool slightly. Squeeze out any excess moisture, then transfer the spinach to a mixing bowl.
Make the filling: Add the crumbled feta, shredded mozzarella, minced garlic, fresh parsley or dill, and black pepper to the spinach. Mix until everything is evenly combined. Taste before adding any salt, as feta can already be quite salty.
Prepare the phyllo dough: Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper. Carefully unroll the phyllo dough and keep the unused sheets covered with a slightly damp kitchen towel to prevent them from drying out.
Assemble the rolls: Place one sheet of phyllo dough on a clean work surface and lightly brush it with melted butter or olive oil. Fold or layer the phyllo as needed, then spoon some of the spinach and cheese filling along one end. Fold in the sides and roll tightly into a neat log.
Repeat: Continue assembling the remaining rolls, placing each one seam-side down on the prepared baking sheet.
Brush and garnish: Lightly brush the tops of the rolls with additional melted butter or olive oil. Sprinkle with sesame seeds, if desired.
Bake: Bake for 20–25 minutes, or until the phyllo rolls are crisp and beautifully golden brown.
Serve: Allow the rolls to cool for a few minutes before serving. Enjoy them warm while the pastry is crispy and the spinach and cheese filling is soft and flavorful.