The peanut butter, with it's thicker texture and protein is obviously the meat, and the jelly, with it's more liquidy texture and lack of nutrition is the condiment
If you've never tried it before, a grilled cheese sandwich cut into your favourite shapes just calls out to be dipped in ketchup. And if ketchup is too much, a creamy tomato soup is also acceptable.
Peanut is a versatile flavor. Sweet, savory, whatever.
You could use it with ketchup if you like. The mix could be a pretty nice combo to use as a sauce for a Thai dish, honestly. Just on bread would probably be pretty overwhelming alone.