Combine the sugar, water, honey, and lemon juice in a medium saucepan. Bring to a simmer over medium heat, stirring until the sugar dissolves.
Simmer for 8 to 10 minutes, until slightly thickened. Remove from the heat, stir in the orange blossom water if using, and let the syrup cool completely.
Layer the baklava:
Heat the oven to 350°F. Brush a 9-by-13-inch baking pan with melted butter.
Stir the pistachios, walnuts if using, sugar, cinnamon, and salt together in a bowl.
Lay one sheet of phyllo in the pan and brush lightly with butter. Repeat until you have 8 buttered sheets on the bottom.
Sprinkle about one third of the nut mixture over the phyllo. Add 6 more buttered phyllo sheets, then another third of the nuts. Repeat once more, then top with the remaining phyllo, buttering each sheet.
Bake and soak:
Use a sharp knife to cut the baklava into diamonds or squares before baking, cutting all the way through to the bottom.
Bake for 40 to 45 minutes, until deeply golden and crisp.
Immediately pour the cooled syrup evenly over the hot baklava. Let it rest uncovered for at least 3 hours before serving, then garnish with extra pistachios.
Notes
Cool syrup poured over hot baklava helps the pastry stay crisp while absorbing flavor.
If your phyllo sheets are larger than the pan, trim them to fit before you start layering.
For a lighter flavor, use all pistachios and skip the walnuts.