Unwrap and carefully unfold thawed phyllo dough; cover with plastic wrap or wax paper and a damp cloth to keep the dough from drying out.
FOR THE FILLING -- Grind the walnuts coarsely, mix with sugar and orange blossom water until well blended. Butter a 12 x 16 inch baking pan. Render the butter.
FOR THE ASSEMBLY -- Layer one package of the phyllo sheets in the pan, buttering generously between each sheet with a pastry brush or cloth dipped in butter. Mix 2-3 tablespoons of water with the walnut mixture and put over the top of the last sheet, making sure to keep the layer of walnuts level.
Repeat the same procedure with the second package of dough, buttering generously between each sheet, and butter the top. Cut into diamonds, dipping the knife into hot water as you go for easier cutting. Pour over any remaining butter.
Bake at 350 degrees for 45 minutes to 1 hour (depending on your oven) or until bottom is done and top is golden brown. If top does not brown nicely, put the baklawa under the broiler for a few seconds, watching constantly, as it will burn very quickly.
FOR THE SYRUP -- Mix sugar and water together and bring to a boil. Boil about 15-20 minutes. Remove syrup from heat, add lemon juice and orange blossom water. Stir and let cool.
SHORTCUT -- Butter pan and place one whole package of phyllo dough in pan, butter top, and add nut filling. Place second package of phyllo dough over nuts and butter the top. Cut into diamonds and pour all the rendered butter into the pan, making sure it settles into all the cuts and around edges. Bake as directed above