Preheat the oven to 300°F (160°C).
In a large bowl, mix the crushed walnuts with the granulated sugar until well combined. Set this aside. Spray the avocado oil into a 12-inch (30-cm) round pan. Place two sheets of the dough into the pan, folding any extra dough inwards. Then spray the top completely with the avocado oil. Place another two pieces of the dough and repeat the process until you have ten filo sheets in total. Spread half of the nut mixture evenly over the base layer of filo dough. Add another eight layers of the dough, spraying every two layers with the avocado oil. Add the remaining half of the nut mixture. Then top it off with twelve more layers of filo dough, spraying avocado oil every two layers. Lastly, spray the top of the baklawa with oil, just before cutting to prevent the top layer from moving. With a sharp knife, cut straight and diagonal slices, all the way through creating diamond shaped pieces. Drizzle the melted butter on top evenly and through all the cuts. Bake for 1 hour and 15 minutes or until the top is golden.
While the baklawa is baking, prepare the sugar syrup. Add the sugar and water to a medium sized saucepan over medium heat. Let it boil until all the sugar has been fully dissolved, around 7 to 10 minutes. Next, add the cardamom pods and let it continue to boil until reduced to two thirds (around 15 minutes). Just before removing from heat, add the lemon juice and set aside to cool completely (30 to 45 minutes). Take the baklawa out of the oven and pour sugar syrup on top while still hot. Let the baklawa cool completely uncovered for around 30 minutes.
Garnish with ground walnuts or pistachios and rose petals, if using.