Foo Fighters -- Now Serving Rock & Roll... With A Slice of Pepperoni

The Foo Fighters just had a CRAZY blowout jam session in Moorpark, CA ... at a pizza joint!

Dave Grohl and FF showed up last night -- unannounced -- at the appropriately named Redballs Rock & Roll Pizza ... surprising 200 serious eaters who paid 10 bucks apiece for the Chevy Metal Christmas Dance Party.

Chevy Metal's a band featuring Foo drummer Taylor Hawkins.

The Foo Fighters and Chevy Metal joined forces on stage for more than 2 hours ... while the audience gorged themselves into a stupor.

This is the first gig for the Fighters in months ... the rehearsals have been downright bloody. (see pic below). They're playing two gigs this week in Mexico City ... where they're Numero Uno.