Grease a 25 cm x 30 cm (10 x 13-inch) baking sheet with butter. Preheat the oven to 190 °C / 375 °F.
Place a saucepan with butter over medium heat. Bring to a boil, then cook for 5 minutes. Remove from the heat. Place a sieve over a bowl, and place a paper towel into the sieve. Pour the butter into the paper-lined sieve to get rid of any impurities. Set aside until needed.TipSkip this step if you're using store-bought clarified butter.
Unroll the thawed phyllo pastry and place it onto parchment paper. Cut the phyllo to the same size as your baking sheet (25 cm x 30 cm (10 x 13-inch). You should get around 24 sheets of pastry. Cover the phyllo dough with parchment paper, then place a damp kitchen towel on top to prevent the dough from drying while you work on your filling.
Stir to combine ground walnuts, cinnamon powder, allspice powder, and nutmeg powder in a bowl.
make the walnut baklava
Add a sheet of phyllo pastry to the prepared baking sheet. Brush with clarified melted butter, then repeat the process until you get 12 layers of pastry. Arrange the walnut filling on top. Continue with the rest of the phyllo pastry. Brush with clarified melted butter, then repeat the process until you get another 12 layers of pastry.TipYou can use any leftover phyllo pastry to fit any empty space on the baking sheet.
cut the baklava
Cut the baklava into four long strips, approximately 5 centimeters (2-inch) in thickness. Pour half of the remaining butter on top and evenly distribute. Using a sharp knife, cut the pastry into 28 diamond-shaped pieces and pour with the rest of the remaining clarified butter on top.
Place a baking sheet with Baklava into the preheated oven on the middle rack. Bake for 30 minutes at 190 °C / 375 °F, then lower the heat to 160 °C / 320 °F and continue to bake for 20 - 30 minutes or until the Baklava is golden-brown and crispy.
While the Baklava is baking, prepare the syrup. Combine sugar, honey, water, cinnamon, and lemon in a saucepan. Place over medium heat and bring to a boil, then lower the heat and let simmer for 10 minutes. Remove from the heat. Stir lemon juice and rose water into the syrup. Before pouring the syrup over the Baklava, remove the lemon slices and cinnamon stick.
While the Baklava is still hot, pour the homemade honey syrup all over the dessert. Set aside to cool for at least 4 hours or overnight. Before serving, optionally decorate with whole walnuts or ground walnuts.