Place walnuts and pistachios into a food processor and process until nuts are well chopped. Pour into a bowl and add powdered sugar, and stir well. Melt butter.
Preheat oven to 375ºF (190ºC). Place a sheet of phyllo in a buttered 13X9 pan.
Butter phyllo sheet. Once buttered completely place another sheet of phyllo on top and repeat buttering process. Continue until you have used half the sheets of dough.
Pour nut mixture into pan and make an even layer. Place a sheet of phyllo dough over nut mixture.
Butter as before alternating with sheets of phyllo. Use up the rest of the sheets of dough.
Using a sharp knife, score top layers of phyllo (into diamond shapes) that sit on top of the nut mixture.
Bake for about 30 minutes or until golden brown.
While baklava cooks make the simple syrup.
Add sugar, water, rose water, orange blossom water and lemon juice, to a cooking pot.
Bring to a boil.
Whisk until sugar has dissolved completely. Remove from heat.
Remove baklawa from the oven and cover evenly with simple syrup.
Cut through all layers of phyllo, remove baklava and serve.