FOR THE FILLING: Place a colander in sink. Bring a 6-quart stock pot filled about half way with water to a boil. Add spinach, stir, cook 1 minute until wilted. Drain spinach in colander; use back of rubber spatula or large spoon to press out excess liquid. Transfer spinach to cutting board and roughly chop. Transfer spinach to clean kitchen towel; squeeze to remove excess water. Place drained spinach in large bowl. Add remaining filling ingredients and mix until thoroughly combined. (Filling can be made up to 24 hours in advance and stored in the refrigerator.)
FOR THE PHYLLO LAYERS: Adjust oven rack to lower-middle position and heat oven to 425°F. Line rimmed baking sheet with parchment paper. Keep phyllo covered with a damp clean towel, otherwise layers will dry and stick together. Using pastry brush, lightly brush 14 by 9 inch rectangle in center of parchment with melted butter to cover area 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 total of 10 layers of phyllo).
Spread spinach mixture evenly over phyllo, leaving 1/4 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 should not be sprinkled with Pecorino).
Working from center outward, use palms of your hands to compress layers and press out any air pockets. Using 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.