Preheat your wood fired oven at about 500°F.
Heat a large cast iron pan in the oven for about 5 minutes.
Add the butter in the cast iron and let it melt in the oven.
Add the bacon, chopped in chunk lenght of about ½ inch each.
Add the celeries and 2 of the green onions and cook for about 5 minutes
Remove from the oven, add the garlic and pour the beer in the cast iron and leave to cook for another 5 minutes or until half reduced.
Remove the cast iron from the oven and drop the mussels in. Cook for another 5 minutes or until all mussels are open.
Just before serving, add some fresh parsley and green onion.
Generously squeeze the juice of the lemon over the mussel and serve.