Mashed up boiled egg with a binder of salad dressing.
Very nostalgic as we had that on long train journeys in the 1940s as there were no catering facilities.
I remember people haveing those sandwiches at school
They were rather well ripe after sitting in a bag against a radiator all morning
one day Brian had some and even he wouldn;t eat them so he lobbed them across the classroom (dunno why we were inside - wasn;t normally allowed)
and they went directly up through a missing ceiling tile and landed somewhere in the dark
wonder if they are still there???
(that was about 1974/5!!!)