It's basically a mince meat sausage, deep fried, smothered with mayonnaise, curry and onions.
There's some herb stuff going on as well but hardly anyone knows the details. Rumors have always gone 'round that it consists of nothing but "butcher leftovers" but that got debunked a while ago. It didn't affect sales in the slightest though.
When my girlfriend was over in Holland, she got slightly addicted to them. My bad, I love 'm too.
.ps: I'm having stomach issues this morning, and this topic (and the other food ones :p) aren't helping. I'm gonna go offline for a bit lol.