Using a blender or food processor, chop the walnuts finely, then pour in the boiling water in a thin stream. Add the feta cheese, then the cream cheese and cottage cheese, paprika, pepper and cayenne, and lemon juice. Continue to blend until smooth, then add the lemon juice and blend a little more. If the spread is a little watery, don't worry; it will solidify as it cools. Turn into a dish, cover tightly and refrigerate for at least an hour before serving.