Melt the butter on the stove while stirring carefully. Once it starts to boil, stop stirring and lower the heat. Boil until the whey evaporates.
Pour 1 tbsp of the melted butter in the oven dish in which you intend to bake the baklava.
Arrange 3 phyllo sheets in it and distribute 2 tbsp of the melted butter on them.
Continue in this order until you've placed half the sheets.
On top, pour in all of the walnuts, blended in a blender, then press lightly.
Continue arranging the rest of the sheets, while pouring on 2 tbsp of the melted butter every 3 sheets.
After you've arranged them, pour on the remaining melted butter over the entire baklava.
Cut the baklava into strips, cut those across to form small rectangles, 1 1/4″ (3 cm) by 1 1/2″ (4 cm).
Bake in a preheated 392°F (200 °C) oven for about 30-40 min. until reddened.
Take the baked baklava out of the oven and leave it to cool for 10 min.
Pour the water in a separate container and add the sugar to it, stirring until it melts.
Once the syrup comes to a boil, add the juice of 1/2 lemon to it and leave to boil another 5 min.
Pour the prepared syrup over the baklava, spoon by spoon.
Cover the syruped baklava with a large container and leave it to sit for a few hours, best if overnight.
Serve it sprinkled with chopped pistachios on each piece.