Justin Bieber & Cody Simpson -- Italian Restaurant Performance Molto Bene (VIDEO)

Justin Bieber and Cody Simpson spiced things up at an Italian joint Monday night ... with an acoustic set.

They showed up at The Nice Guy in West Hollywood at around 9 PM, and treated the dinner crowd to their 2014 duet, "Home to Mama," the Edwin McCain song "I'll Be" and Tracy Chapman's classic "Fast Car."

Bieber bailed after 30 minutes but Cody lingered for a while longer.