Pulse the walnuts in a food processor until finely chopped or coarsely ground. Pour into a medium-size bowl.
Pulse the pistachios in the food processor until finely chopped or coarsely ground. Add to the bowl with the walnuts. Stir in the sugar, ground cinnamon, and ground nutmeg.
Preheat the oven to 350 degrees F. Prepare a 9×13 baking pan by brushing some of the melted butter over the bottom.
Trim the phyllo dough to fit the baking pan as best as you can. I use Athenos brand and each package has 2 rolls of 40 sheets measuring 9×14 inches. So I trim them slightly, by about 1-inch.
Keep the phyllo dough covered with a damp towel while not in use. You don’t want it to dry out and start breaking.
First Layer – Lay 10 sheets of phyllo dough into the pan one at a time, brushing each sheet with the melted butter before placing the next sheet. Sprinkle about 3/4 cup of the nut mixture evenly over the layers of phyllo dough.
Second layer – Place 5 sheets of phyllo dough brushing each one with butter. Sprinkle another 3/4 cup of nut mixture.
Third layer – Place 5 sheets of phyllo dough brushing each one with butter. Sprinkle another 3/4 cup of nut mixture.
Fourth layer – Place 5 sheets of phyllo dough brushing each one with butter. Sprinkle another 3/4 cup of nut mixture.
Fifth layer – Place 5 sheets of phyllo dough brushing each one with butter. Sprinkle another 3/4 cup of nut mixture.
Final layer – Lay 10 sheets of dough buttering each one. Brush the very top layer with the butter again.
To cut the baklava into diamonds, using a sharp knife cut into about 1 1/2″ wide strips (three cuts). Then cut diagonally into diamond shapes.
Bake uncovered for 35 to 40 minutes, or until the top is golden brown.
Remove from the oven and immediately pour or spoon the cooled syrup over the hot baklava. Garnish with finely chopped pistachios or walnuts if desired.
Let the baklava cool completely on a wire rack, uncovered at room temperature. For the best results cover once it is completely cooled and let it sit overnight.