Preheat your oven to 350°F (175°C). Lightly grease a 9x13-inch baking pan.
Prepare the syrup first so it has time to cool: combine Granulated Sugar and Water in a small saucepan, bring to a boil, then reduce heat and simmer until slightly thickened (about 8–10 minutes). Stir in Lemon Juice, remove from heat and let cool completely.
Coarsely chop and lightly toast the Pistachios in a dry skillet for 2–3 minutes until fragrant, then set aside to cool.
Unroll the Phyllo Dough Sheets and keep them covered with a damp towel so they don’t dry out. Have the melted Clarified Unsalted Butter ready for brushing.
Place one sheet of phyllo in the prepared pan and brush lightly with melted butter; repeat layering and buttering about 6–8 sheets for the bottom base.
Evenly sprinkle a layer of the chopped pistachios over the buttered phyllo, then continue layering phyllo sheets, brushing each with butter, and adding pistachios every 2–3 sheets until you use most of the nuts — finish with about 6–8 buttered phyllo sheets on top.
Using a sharp knife, cut the assembled baklava into diamonds or squares all the way through to the bottom of the pan before baking.
Bake in the preheated oven until the top is deep golden and crisp, about 35–45 minutes, rotating the pan halfway if needed for even browning.
As soon as the baklava comes out of the oven, pour the cooled lemon syrup evenly over the hot pastry so it soaks into the layers. Allow the baklava to cool completely at room temperature (at least 4 hours or overnight) so the syrup is fully absorbed and the layers set.
Serve at room temperature, optionally garnished with a sprinkle of extra chopped pistachios for color and crunch.
