Persillade: in a bowl, mix all the ingredients and season generously.
Mussels: in a large pan, bring about 375 ml (1½ cups) of water to a boil and add the mussels. Cover and cook until they open (toss those that stay closed). Remove the top shell. Place the mussels side by side on a baking sheet and top with persillade. Place the sheet in the oven and cook until the top is golden.