Preheat oven to 375F. With a food processor or by hand, cream the butter with the tahini. Add the salt and brown sugar. Blend until smooth. Sprinkle in the flour, blending well. Mix in the chopped nuts. The dough will be very stiff. Lightly butter two 7-inch pie plates or shallow baking pans.
Press the dough to evenly cover the bottom and sides of the pie plates to a thickness of no more than 1/4 inch. Press a few whole nuts into the surface to decorate. Bake the shortbread for 15 minutes and then check it frequently, every couple of minutes, and remove it from the oven as soon as the edges are golden brown. Be careful not to overbake it.
While it's still warm, cut each shortbread into 8 or 10 wedges in the pan; don't wait until it's cool or it will crumble.