
Key Lime Pie was invented by American settlers of Florida to use what they had to hand - fresh-laid eggs, tinned condensed milk and fresh key limes. The original pie filling used nothing more than this and relied on the acidity of the lime juice to coagulate the eggs and milk alone.
In these days when we get our eggs from the supermarket of an unknown age, modern recipes lightly cook the pie in order to ensure the raw egg yolks are pasteurised.
This variation bulks out the condensed milk filling with some cream cheese, which makes it a little less sweet, and a bit more cheesecake-like.