Carefully separate filo dough. Trim dough to fit in the pan and cover with a damp cloth.
Grind together walnuts and almonds, then add pistachios. Beat egg whites until stiff, then add honey, nuts, cinnamon and rosewater.
Grease a baking dish with butter. Layer sheets of dough with nut mixture, brushing each sheet of dough with butter. End with a sheet of dough.
Bake in an oven preheated to 200°C (approximately 400°F) for 30 minutes.
Meanwhile, boil sugar and 250 ml (approximately 1 cup) of water in a pot for 1 minute. Remove from heat, let cool slightly and stir in lemon juice.
Remove baklava from oven and drizzle lemon syrup over baklava while still warm. Let cool and sprinkle with grated pistachios. Slice and serve.