Stir together ground almonds, sugar, flour, cinnamon, and lemon peel in a large mixing bowl. In a separate bowl, combine egg, egg yolk, and melted butter. Add to ground almond mixture, stirring until dry ingredients are moistened. Divide dough in half.
Roll or pat each half into 12x8-inch rectangles on a light greased and floured baking sheet. Brush each portion of dough with egg white, and sprinkle with half of the sliced almonds. Cut rectangles into diamond shapes or squares, but do not separate the pieces.
Bake in a 325 degree F oven about 20 to 22 minutes or until dough and almonds on top are lightly browned. Cool on pan for 5 to 10 minutes. Cut along score marks. Transfer to wire racks and cool completely. Makes 70 to 80 cookies.