Originated from Andy Griffith (speaking on a Ritz Cracker TV commercial):
"Mmm-mmmmh. Anything sits good on a Ritz." Supposed to mean that Ritzes are so excellent -- ritzy, even.
IT SITZ ON THE RITZ refers to being good enough for a Ritz (figuratively).
Not limited to just food>
"This Humboldt Purple I gotz here SITS ON THE RITZ. Here, load a bowl and see for yourself."
"Oh, that's just my cuz. He OK; he SHITS ON THE RITZ."