Preheat oven to 350 degrees. Melt butter in a medium saucepan over medium heat, skim off foam and trasnfer butter to a small saucepan, discarding solids. Using a pastry brush, butter a large baking dish (about 11" by 15" by 2") with some of the butter and set aside. Put walnuts and 1 cup of the sugar into the bowl of a food processor and process until finely ground, about 30 seconds. Set filling aside.
Unwrap phyllo and cover with a clean, damp dish towel to keep sheets from drying out. Uncover stack, carefully separate top sheet of phyllo from the stack, lay flat in prepared baking dish and run any overhang up sides and over rim of dish. Quickly cover stack again with dish towel. Brush sheet of phyllo in dish with some of the melted butter, lay a second sheet of phyllo over the first and brush with some more of the butter. Repeat layering process, warming the butter over low heat if it begins to thicken, until half the phyllo, about 14 sheets, has been used.
Spread reserved filling evenly over buttered phyllo. Lay 1 sheet of phyllo out on a clean surface, brush with butter and place (buttered side up) over filling (nuts make this layer hard to butter in dish). Continue layering as above using remaining phyllo. Brush top with butter. Using a sharp knife, trim over hanging phyllo. Cut baklava on the diagonal at 2" intervals in a diamond pattern, down to the bottom of dish. Bake until golden brown, about 45 minutes.Meanwhile, put the remaining 2 cups of sugar and 1 cup water into a medium saucepan over medium heat and simmer, stirring often, until sugar dissolves, about 5 minutes. Remove pan from heat and stir in lemon juice and orange flower water (if using). Remove Baklava from oven and pour hot syrup over entire surface, tilting dish so syrup reaches all corners. Set aside to let cool to room temperature.