I've had stuffed mussels in a french restaurant in Washington DC (but not when I was at a french restaurant in France--so maybe not so genuine?) Anyway, I remember really liking them and when I was coming up with appetizer ideas I decided to do a take on oysters rockefeller.
I like this preparation a lot. I made a more refined version that I brought to my sisters house but I did not like them as much. For those I put the ingredients (minus the mussel) into a food processor. The result was a finer stuffing that easily packed the shell of the mussel but the flavor wasn't as good. I like what roughly chopping the herbs, greens and dried bread does to the texture of these.
Decide for yourself what kind of preparation suits you, if you put all the ingredients in a processor then the stuffing will have a more green appearance overall. Chopping the greens distributes them differently and the bread crumbs and dried bread stand out more.
Also, I used more of the herbs de provence blend when I brought these to my sisters. I would say stick with the amount I have below--using more was a bit overpowering. Unless you use only chives and parsley or tarragon which have a less pronounced flavor.
This recipe makes 12 and is easily doubled, tripled, etc...