I believe the Dutch first made the idea of Fries and Mayo popular though the mayo and ketchup mix I have no idea where it originates from however I do know, that if you mix mayonaise and ketchup in a roughly even ratio you get what is often called burger sauce over here (some argue it tastes even better than actual burger sauce but I know when I am ordering chips drunk from the chip shop opposite the pub on the way home with my mates I tell the guy to pass the burger sauce not the ketchup and mayo) Either way it's really nice, another nice mayonaise and other condiment mix is mayonaise and mustard though this is sold commonly as Djonaise sauce (or something similar)
Interesting food combinations I eat? Well lots of people look at me like I was silly if I mention I love cheese and grapes, it doesn't just need to be brie for me to put grapes in with it just mature/extra mature cheddar will do for me. I also like chicken and chorizo on a pizza (god bless the girlfriend bringing me pizza from work! :D) also prawn and chorizo pizza is surprisingly nice. Other than this, Cheese bread (bread baked with cheese in the mix for those who think wtf?) with humous is quite nice though it shouldn't go. And when I was a kid I always used to have pickled onion monster munch (a type of chips/crisps for americans or the ignorant) with mars bars surprisingly they're really nice together, throw a can of cherry coke for the ultimate in sugary fatty snacks. Ham and Thai Sweet Chilli Sauce sandwiches are good but taste better when you add peppered german salami and some light salad to the mix. And an alternative to sausages in the traditional "fry up" breakfast (this only works because I don't eat bacon) is to use fish fingers I regulary swap my sausages for fish fingers at work even with fried egg and hashbrowns it's nice. I'm surprised I aint 4 times my current size in terms of fat and weight!