Preheat oven to 350° F. Grease cookie sheet lightly. Set aside.
Combine flour, cashew, sugar and lemon rind. Cut in margarine until it resembles coarse crumbs. Beat the egg yolk and lemon juice. Add to flour mixture. Knead until smooth.
Roll out dough to 1/2" inch thick then cut into rounds or use Christmas cookie cutters.
Place on prepared cookie sheet. Slightly beat egg white. Brush it over cookies. Sprinkle cashew, sliced almond or colored sugar. Bake for 15 minutes or until lightly brown (do not over bake). Loosen from sheet while hot and cool on a rack.