Preheat the oven to 350 degrees Fahrenheit.
First make the simple syrup. Add all the ingredients to a small pot and bring to a gentle simmer until all the sugar is dissolved and it thickens slightly, for about 5-8 minutes. When done, set aside to cool.
Make the Pistachio Filling. In a food processor, add the pistachios, cardamom, cinnamon and lemon zest. Pule until the pistachios are finely ground but not too powdery. Set aside.
Layer the phyllo. Unroll the thawed phyllo dough and place a damp towel on top as you work with it. Brush a 9 x 13 baking pan with melted butter on all sides and layer two sheets of phyllo, then brush with butter and continue until half of the sheets are used.
Add the pistachios. Pour the pistachio mixture onto the phyllo and reserve a few tablespoons to garnish the top of the finished baklava. Spread out the pistachios in an even layer.
Continue layering two sheets of phyllo and brushing with the melted butter until all of the phyllo dough is used up. Once layered, brush the top with melted butter.
Cut the baklava into squares before baking. Use a sharp knife to cut the baklava into 24-26 squares.
Bake the pistachio baklava at 350 degrees Fahrenheit for about 22-25 minutes, until the top is golden brown and crisp.
Pour the syrup over the hot baklava. Once the baklava is cooked, remove it from oven and pour the orange blossom simple syrup all over the top, while the baklava is still hot.
Allow the pistachio baklava to rest for at least 2-3 hours before serving so the baklava can soak up all of the syrup. Garnish the squares with the remaining ground pistachios.