The tenderest and best tasting mussels are the small ones (about 4cm in length) with blue-black shells, often labelled bouchot (after they way they're raised). Don't use the 'green-lipped' variety for this dish, as they're too large and coarse.
This dish is quite versatile. You can add leeks or fennel (chop into small pieces then cook with the butter, garlic and shallots until tender before stirring in the mussels); add a splash of Pernod or other anise-flavoured spirit; or omit the white wine and stir in a spoonful of commercially prepared Thai green curry paste and some coconut milk, just before you add the mussels.
1kg fresh mussels
30 grams unsalted butter
4-6 shallots, thinly sliced
1-2 garlic cloves, thinly sliced
150ml dry white wine
Roughly chopped Italian parsley
Fine sea salt and freshly ground black pepper
Check the mussels carefully and discard any with cracked shells. Rinse them under cold running water and remove the tough, fibrous 'beards' if they have any (cultivated mussels rarely do). Drain well.
Melt the butter in a wide, deep pot, add the shallots and garlic and cook over a low flame until soft.
Turn the heat to medium and add the white wine.
Bring to the boil and cook for one minute, then add the mussels.
Let the liquid boil again, then lower the heat, cover the pan with a lid and simmer.
Shake the pan frequently and cook until the mussels open, about five minutes.
Discard any mussels that haven't opened.
Sprinkle in some black pepper then taste the sauce and if needed, add some salt (it might not need any because the mussels are salty).
Stir in the chopped parsley.
Ladle the mussels and cooking liquid into wide bowls and serve with thick slices of toasted bread.