Preheat the oven to 350°F. Brush the bottom of a 9 x 13 pan with ghee.
In a large bowl, stir together the pistachios and granulated sugar until well combined.
Unroll the two sleeves of phyllo sheets and cut off an inch at the end to make them fit into the 9 x 13 pan.
Lay one sleeve (20 phyllo sheets) in the prepared pan. Add about 10 more phyllo sheets on top from the second sleeve. Spread the nut mixture over the phyllo in one even layer. Lay the remaining phyllo sheets over the nuts.
Pour the melted ghee over the baklava and tilt the pan carefully to spread it all over. Cut the baklava into diamonds by making 4 cuts lengthwise and 9 cuts crosswise on the diagonal.
Bake until golden brown all the way into the cut lines, about 45–50 minutes, rotating the baklava halfway through.
Meanwhile, make the simple syrup by combining the water, sugar, and lemon juice in a small saucepan over medium-high heat. Bring mixture to a boil, then reduce heat to simmer, stirring occasionally until the sugar dissolves and the mixture thickens but is still clear, about 5–7 minutes. Remove from heat, add the orange blossom water, and set aside to cool.
When the baklava finishes cooking, remove the pan from the oven and immediately pour the cooled simple syrup evenly over top. Cool at room temperature for at least 2 hours before serving.