Caramelized onions are, sure. But i'm assuming the 4chan poster bit into a raw onion with a Carmel glaze, however, which does not seem appetizing to me.
One Christmas extended family had a Christmas gift exchange, one uncle had made a "gag gift", it was a piggy bank with pennies, I got it, he felt bad but to everyone's suprise, I was estatic. I was 5 and collecting coins, a piggy bank filled with loose change was the best present.
I think the apples that are typically used are the sour varieties so that there is a flavor contrast when biting in to it. This is just what I heard though, I usually don't like whatever is covering them.