For the syrup: In a heavy saucepan combine the sugar, honey, 1/2 cup water, cinnamon stick and salt. Squeeze the lemon over the saucepan before adding it. Place the pan over medium-high heat and bring the mixture to a boil; cook, stirring occasionally, until the sugar is dissolved. Remove from the heat and cool to room temperature. When the syrup has cooled, remove the cinnamon stick and lemon. Chill the syrup in the refrigerator for at least an hour and up to 3 days.
For the filling: In a large bowl, combine the almonds, walnuts, pistachios, sugar, cinnamon, nutmeg, cloves, cardamom and salt and toss to combine. Transfer 3 tablespoons of the mixture to a small bowl.
Preheat the oven to 350 degrees F. Brush a 9-by-13-inch glass baking dish with melted butter.
Halve the phyllo sheets crosswise and stack the sheets on top of each other. Cover the stack with a clean damp kitchen towel. Lay 3 sheets of phyllo in the bottom of the baking dish and brush the top sheet generously with butter. Continue to layer 3 sheets at a time, staggering the sheets to cover the bottom of the dish evenly and brushing the top of every third sheet generously with butter, until you have used 12 sheets of phyllo.
After brushing the top layer with butter, sprinkle one third of the nut mixture from the large bowl over the top. Drizzle with 2 tablespoons of the butter. Repeat with 12 more sheets of phyllo, another third of the nuts, another 12 sheets of phyllo and the final third of the nuts, brushing with butter on the top of each phyllo sheet.
Top with 10 sheets of phyllo, brushing the top of each sheet with butter. Drizzle 3 tablespoons of the butter over the top sheet. Using a serrated knife, carefully cut the baklava into 16 equal rectangles, then cut each piece in half diagonally.
Bake the baklava until golden brown and crisp, about 45 minutes. Transfer the dish to a rack. While the baklava is still hot, slowly pour the cold syrup around the edges of the pan, in between all the cuts and over the top. Garnish with reserved 3 tablespoons of nut mixture. Let the baklava stand at room temperature for at least 6 hours to allow the flavors to develop. Store covered at room temperature for up to 10 days.