Which episode was it ... ? [spoilers?]

Trying to find/remember the name of the episode that included this exchange: Luke is sick and Goth God sends Joan to the store to pick up cream of wheat for him. Joan pitches a fit, saying that she's tired of doing what He asks. He chuckles and says something along the lines of, "Out of all the assignments I've given you, you're refusing to buy cream of wheat?" To which she responds, "Because it's endless!"

Anyone know?

Thanks!