Preheat oven to 350°. For filling, in a food processor, combine the cashews, walnuts, sugar, cardamom, cinnamon and allspice. Cover and pulse until nuts are finely chopped. Brush a 13x9-in. baking pan with some of the butter. Unroll phyllo dough; trim to fit into pan., Layer 4 sheets of phyllo dough in prepared pan, brushing each with butter. (Keep remaining dough covered with a damp towel to prevent it from drying out.) Sprinkle with a third of the nut mixture. Repeat layers twice. Top with remaining phyllo dough, brushing each sheet with butter., Using a sharp knife, cut baklava into 6 squares, then cut each square into 4 triangles., Meanwhile, in a large saucepan, combine the syrup ingredients. Bring to a boil. Reduce heat; simmer, uncovered, for 10 minutes, stirring occasionally. Discard lemon slices and cloves. Pour over warm baklava. Cool completely on a wire rack. Cover and let stand overnight.