Preheat oven to 180°C.
To make the syrup, combine sugar and water in a small saucepan over low heat. Cook, stirring, until sugar dissolves. Increase heat to high and bring to the boil. Add honey. Cook until syrup thickens slightly. (I like to add a dab on the stove and test the consistency with my finger, if it feels like it has body to it, it's done) Remove from heat. Add Iranian rosewater and set aside.
Place pistachios in a food processor and process until finely chopped, remove to a bowl. Add the almonds and process until finely chopped. Transfer to a bowl. Place the walnuts in food processor and process until desired consistency. Mix all nuts with freshly ground cardamom, cinnamon and 1/2 cup sugar, stir to combine.
Grease 9X13 pan. Open filo pastry and cover with the sheet provided and then and damp cloth on top to prevent drying out while working. Cut sheets as needed as you go. You may need to cut each sheet in half. Place a sheet of filo pastry cut to the size of the pan on a clean work surface and using a pastry brush, brush with a butter. Continue layering with another 6 sheets of filo and butter. Trim edges to fit base of the pan if needed. Sprinkle with half the nut mixture. Repeat process with half the remaining filo and butter. Continue layering with filo and nut mixture. Top with remaining filo. Use a small, sharp knife to cut into diamond shapes. (find out how by looking online as they look prettier that shape. Bake in preheated oven until golden brown 45 minutes - 1 hour. Remove from oven and, while hot, pour over as much as needed, COOLED syrup. Set aside to cool.
Top with slivered pistachios and rose petals to decorate.
Baklava is better made a day before serving for texture and taste.