I checked out your restaurant's website.... wish that I could visit to diagnose the problem in person!
You didn't say specifically what is troubling the Mondiale, however, the
Getting Started section of the
Resources page is my recommended reading, especially
How I Stopped Worrying and Learned to Love HXs. Compare the guidelines in that writeup and that of the
Cimbali Junior's and you'll see that the flush routine is quite different. I know it caused me a bit of confusion going the opposite direction (E61 / HX to Cimbali Junior), i.e., I was overflushing the Junior because of my prior experience with espresso machines like the Isomac Mondiale. It's my guess that you're underflushing and getting bitter espresso.
PS: David Morris is an HB member (beta14ok) and Mondiale owner; if you have questions specific to the Mondiale, he can help you too.