To make the simple syrup, combine the water, sugar and lemon juice in a small saucepan over medium-high heat. Bring mixture to a boil, then reduce heat to simmer, stirring occasionally until the sugar is dissolved and the mixture is thickened but still clear colored, about 5-7 minutes. Remove from heat, add the orange blossom water, and set aside to cool to room temperature.
In a large bowl, combine the walnuts and granulated sugar until well mixed.
Preheat the oven to 350°F. Brush the bottom of a 9 x 13 pan with clarified butter.
Open the two sleeves of phyllo sheet - each will have about 20 sheets. Unroll them and cut off an inch at the end to make them fit into the 9 x 13 pan.
Lay one stack of 20 phyllo sheets in the pan. Add about 5-10 more phyllo sheets on top of the first stack. Spread the nuts mixture over the phyllo in one even layer. Lay the remainder 10-15 phyllo sheets over the nuts.
Cut the baklava into diamonds by making 4 cuts lengthwise and 9 cuts crosswise on the diagonal, holding down the phyllo sheets with your other hand as you cut carefully.
Pour the melted clarified butter over the baklava evenly. Allow the butter to settle in, about 5 minutes.
Bake until golden brown all the way into the cut lines, about 50-60 minutes, rotating the baklava halfway through baking.
Remove the pan from the oven and immediately pour the cooled simple syrup evenly over the baklava. Allow to cool at room temperature for several hours before serving.