Heat your gas or charcoal grill to medium-high heat. When i'm on the move, i like using my Nomad portative grill
While the grill heats, gather your mussels;
Clean the mussels from any dirt, remove the beards and throw away any mussels that don't close tightly when you tap on it
Add your fresh Rosemary on the grill grates and put the cleaned mussels on top. Close your grill until the mussels opens, it should take about 5 minutes.