A good bit of thick blue sirloin steak, with onions in a buttered, soft baguette cut in half.
Either that, or a monstrosity creation which I don't think abides by the laws of physics, or the laws of Peru, which I have had once before...
A BIG bit of soft brownie, with vanilla ice cream and toffee sauce, sandwiched between two giant chocolate chip cookies. (I dont think this counts as a sandwich, but it is delicious)
I don't actually live in Peru or have ever been there, It was just for comedic effect
EDIT: everything here can be improved with bacon, including Wadders' idea