Preheat the oven to 180°C, 350°F, Gas 4.
Melt the butter in a pan. Lay a sheet of filo pastry in a greased baking tray and brush with butter. Repeat with half the filo sheets, brushing each one with melted butter before laying another on top.
Mix together the nuts, sugar and cinnamon in a bowl then sprinkle evenly over the filo pastry. Lay the remaining filo sheets on top, brushing each one with butter as before. Cut a diamond pattern into the top pastry sheets, then bake in the oven for about 20 minutes.
Reduce the heat to 160°C, 320°F, Gas 3 and bake for another 25-30 minutes until the pastry is golden on top and slightly puffed. If necessary, cook for a further five minutes.
Meanwhile make the syrup: heat the sugar, water, a little lemon juice and blossom water if using in a pan until the sugar has melted. Do not stir. Remove the baklava from the oven and set aside. Pour the syrup evenly over the pastry and leave to cool. Cut into diamond-shaped pieces before serving.