Place the cleaned mussels in the pot of an Old Dutch and set aside.
Lay the bacon in the cold Old Dutch Lid, then place it over high heat.
Cook until sizzling, browned and crisp.
Transfer the bacon to paper towels to drain, leaving the rendered fat in the lid.
Add the olive oil, leek, onion, garlic and bay leaves.
Cook until the leek and onion have softened and are beginning to brown.
Pour in the cider and simmer for 5 minutes, allowing it to reduce slightly.
Roughly chop the bacon and return it to the lid.
Simmer for another minute.
Pour the bacon and cider mixture over the mussels.
Add another splash of cider, cover with the lid and place over high heat.
Steam for 5–10 minutes, until the mussels have opened.
Discard any mussels that remain closed.
Finish with lemon juice, parmesan, salt and black pepper.
Serve immediately with the broth from the bottom of the pot.