Prepare the syrup: Heat sugar and water until dissolved. Add lemon juice and simmer for 10 minutes. Cool completely.
Prepare the nuts: Chop pistachios finely and set aside.
Layer the phyllo: Brush a baking dish with butter. Place 10 layers of phyllo, brushing each with butter. Spread half the nuts evenly, add 10 more buttered layers, spread the remaining nuts, and finish with 10 to 12 buttered layers.
Cut before baking: Slice into diamonds or squares using a sharp knife.
Bake at 170°C for 45 to 50 minutes until golden and crisp.
Add syrup: Pour the cooled syrup evenly over the hot baklava. Let it rest at least 6 hours, preferably overnight.
Serve: Garnish with crushed pistachios. Serve with Turkish tea or coffee.