Not a parasite; most closely resembles a slice of onion/onion skin. If you do an internet search for sauteed onions, you will find many beautiful (and tasty looking) examples of similar-appearing structures.
This case is a great reminder that many food partially-digested objects can be seen in human stool specimens, and many of them (e.g. bean sprouts, bubble gum, citrus pieces, onion slices, tomato skins) can be seen an mistaken as parasites. The definitive way to differentiate these objects from true parasites is to look them under a dissecting microscopy and by light microscopy if necessary, examining the consistency and looking for eggs (as suggested by Michigan Micro).