Clarify the butter in a heavy saucepan or skillet in the following fashion: Melt the butter slowly over low heat without letting it
brown, skimming off the foam as it rises to the surface. Remove the pan from the heat let it rest for 2 or 3 minutes, then spoon off the clear butter and discard the milky solids at the
bottom of the pan.
Preheat the oven to 350° and stir the vegetable oil into the clarified butter. Using a pastry brush coat
the bottom and sides of the 13x9-inch baking dish with about 1 tablespoon of the butter mixture. Fold a sheet of phyllo in half crosswise, lift it up gently and unfold it into the prepared
dish. Press the pastry flat, fold down the excess around the sides and flatten it against the bottom.
Brush the entire surface of the pastry lightly with butter and oil mixture, and lay another sheet of phyllo on
top, oil mixture, and lay another sheet of phyllo on top, folding it down and buttering it in similar fashion. Sprinkle the pastry evenly with about 3 tablespoons of walnuts.
Repeat the same procedure using 2 sheets of buttered phyllo and 3 tablespoons of the walnuts each time to make
19 layers in all. Spread the remaining sheets of phyllo on top and brush the baklava with all the remaining butter and oil mixture.
With a small, sharp knife score the top of the pastry with parallel diagonal lines about 1/2 inch deep and 2
inches apart, then cross them diagonally to form diamond shapes. Bake in the middle of the oven for 30 minutes or until the top is crisp and golden brown.