Stir the fresh blue mussels in a colander while rinsing in tap water. Set aside for a few minutes. Tap any that are open and discard those that don't close in response to the tap.

Heat oil in a large pan over medium-high heat. Add pancetta, prosciutto or bacon and cook, stirring occasionally, until crispy. Add shallot and cook, stirring, for two to three minutes more. Add wine, lemon juice and blue cheese. Stir briefly, then add mussels.

Cover and leave to steam for five to seven minutes, or until mussels are open. Discard any mussels that do not open.

Toss in greens and salt and pepper to taste. Transfer to a serving platter and garnish with lemon zest.

Serve with slices of crusty bread.