Preheat the oven to 350°F (180°C) with a convection setting, if available. Grease a 9x13-inch baking dish with a bit of the melted butter.

Lay one package of phyllo dough in the pan, trimming to fit if needed. Spread the ground pistachios evenly over the dough, pressing lightly to create a uniform layer.

Place the second package of phyllo dough on top of the pistachios. Gently press down with your hands to compact the layers. Using a sharp knife, carefully score the baklava into small squares or diamonds.

Slowly pour the melted butter over the baklava, making sure it seeps into the cuts. Use the back of a spoon to spread the butter, ensuring it coats the surface and drips between the layers.

Transfer the pan to the oven and bake until the baklava is deeply golden and crisp, 25–30 minutes.

While the baklava bakes, combine the water, sugar, and lemon juice in a small saucepan. Bring to a boil over medium heat, stirring occasionally until the sugar dissolves. Reduce the heat to low and simmer for 10 minutes, until slightly thickened.

Assemble and serve:

As soon as the baklava comes out of the oven, pour the hot syrup evenly over the top. Let the baklava sit at room temperature for at least 1 hour to absorb the syrup and cool.

Garnish with more ground pistachios or crushed nuts, if desired. Serve at room temperature or slightly chilled.