To make the Sugar Syrup:
Combine sugar, water, glucose syrup (if using), lemon, and salt in a small saucepan. Bring to a boil over medium-high heat, stirring until sugar dissolves. Remove from heat and stir in orange blossom water.
Pour into a measuring cup with a spout or gravy boat and let it cool completely. Cooled syrup can be refrigerated for up to 4 days; bring to room temperature before using.

To make the Pastry:
Preheat your oven to 300F (150C) and adjust the oven rack to the lower-middle position.
Pulse pistachios in a food processor until very finely chopped. Transfer to a bowl.
Brush a 13X9-inch baking pan with ghee or clarified butter. Unwrap and unfold the phyllo sheets on a cutting board. Trim the phyllo to fit the pan and cover with plastic wrap, then a damp kitchen towel to keep it from drying out.
Place one sheet of phyllo in the prepared pan and brush with ghee or clarified butter. Layer 7 more sheets, brushing each with more ghee/butter. You will have 8 phyllo layers. Spread 1 cup of the chopped pistachios evenly over the top.
Layer 6 more phyllo sheets, brushing each with ghee/butter, and top with another cup of pistachios. Repeat this process with 6 more phyllo sheets, more ghee/butter, and the remaining cup of pistachios.
Layer the final 10 phyllo sheets, brushing each with ghee/butter except for the very top layer. Gently press down on the layers with your palms to remove air pockets. Cut the baklava into diamond shapes using a sharp knife. Brush any remaining ghee/clarified butter over the surface.
Bake for 1 to 1 1/2 hours, until golden and crisp. Rotate the pan halfway through baking.
Immediately pour the cooled syrup over the cut lines, reserving about 2 tablespoons. Drizzle the remaining syrup over the top. Sprinkle a pinch of ground pistachios in the center of each piece.
Let cool on a wire rack for about 3 hours. Cover with foil and let stand for at least 8 hours before serving. Baklava can be stored tightly wrapped in foil at room temperature for up to 10 days.

To Clarify the Butter (if using instead of ghee):
Slowly melt the butter over medium-low heat until the milk solids separate and collect at the bottom. Remove from heat, let settle for 10 minutes, then carefully skim off the foam from the surface. Pour the clear butterfat into a bowl, leaving the milk solids behind. You should have about 1 1/8 cups of clarified butter.