Arrange the rolled out sheets into 3 stacks, 10 per stack, while sprinkling generously with starch in between each.
Roll out each stack into a single sheet to a diameter of 14 1/2″ (36 cm).
Put this larger sheet in the oven dish and evenly distribute the 1st cup of ground walnuts. Roll out the 2nd stack into a sheet and put it on top of the spread out walnuts, then distribute the 2nd cup of walnuts over it.
Cover with the 3rd sheet and cut the baklava into rhombuses. Melt the butter and pour it evenly over the cut baklava. Bake in a preheated 320°F (160 °C) oven for about 1 hour.
In a pot, combine the water and sugar for the syrup, put it on high heat to boil. Once it comes to a boil, lower the heat and leave it to simmer for 10 min., add the lemon juice.
Pour the hot syrup over the cooled baklava, cover with aluminum foil and leave it to cool.