Arrange the phyllo pastry sheets carefully in a buttered oven dish, one atop the other, without adding any oil in between them. Distribute the walnuts evenly in the middle in 2 spots (not at the very top).
Cut it into rhombuses. Melt the butter (without letting it boil) and pour evenly, while still hot, over the baklava. Bake in a moderate oven until golden. Leave it to cool completely.
Make a syrup from the sugar and water, let it boil for 5 min. while stirring nonstop. Finally, add the juice of 1 lemon and a little lemon zest to the syrup.
Pour it over the baklava and leave it to sit overnight to soak it in well. Enjoy!