Adjust oven rack to lower-middle position and heat oven to 425 degrees.
Line rimmed baking sheet with parchment paper.
Using a pastry brush, lightly brush 14 by 9-inch rectangle in the center of parchment with melted butter to cover an area the same size as phyllo.
Lay 1 phyllo sheet on buttered parchment, and brush thoroughly with melted butter.
Repeat with 9 more phyllo sheets, brushing each with butter (you should have a total of 10 layers of phyllo).
Spread spinach mixture evenly over phyllo, leaving a ¼-inch border on all sides.
Cover spinach with 6 more phyllo sheets, brushing each with butter and sprinkling each with about 2 tablespoons Pecorino cheese.
Lay 2 more phyllo sheets on top, brushing each with butter (these layers should not be sprinkled with Pecorino).
Working from the center outward, use palms of your hands to compress layers and press out any air pockets.
Using a sharp knife, score pie through the top 3 layers of phyllo into 24 equal pieces.
Sprinkle with sesame seeds (if using).
Bake until phyllo is golden and crisp, 20 to 25 minutes.
Cool on baking sheet 10 minutes or up to 2 hours.
Slide spanakopita, still on parchment, to cutting board.
Cut into squares and serve.