Egg Mayonnaise and Cress Sandwich

Recreating a fond memory from being poor in London

Though egg mayonnaise is essentially the same thing as American egg salad, it doesn't taste like your average pitch-in.   The mayonnaise was creamy but it had a lightness to it, which probably has something to do with the proportion of ingredients.  Instead of deli-style New York sandwiches where a literal pound of meat is thrown on each sandwich--"It's like a cow with a cracker on either side,...