Grease and line bottom as well as sides of an 8" square tin. Lightly toast the nuts. Cool.
Beat egg whites till foamy. Add castor sugar a spoonful at a time. Beat until stiff and glossy, but not dry. Add sifted flour and nuts. Spread into tin and smooth the surface. Bake in 170C oven for 30 minutes. Cool in tin. Cut into slices with serrated knife. Line cookie tray with paper. OVen 150 C. Bake 15 minutes, then turn over the slices, bake another 15 minutes. Leave longer at a lower temp if they are not hard and crispy enough.