Beat egg yolks and combine with condensed milk. Add lemon rind, juice, vanilla and nutmeg. Blend well.
Fold in stiffly beaten egg whites. Set aside. Combine melted butter and zweiback crumbs. Sprinkle butter 20 cm square or 23 cm round cake pan with half the crumbs. Pour in reserved filling mixture and sprinkle with remaining crumbs. Bake at 160ºC ofr 30 minutes.
Turn off heat;let cool for one hour in oven with door closed. Makes 8 servings.