Step 1 – Wash and clean mussels. Add white wine and chopped onion to a saucepan and bring to a boil. Add mussels and cook on medium heat for 5 minutes or until the mussels open, then set aside.
Step 2 – To make the green sauce, place the remaining chopped onions in a bowl and rub them with the sugar. Wash with lots of cold water, then drain in a colander.
Step 3 – Place drained onions, chopped parsley and lemon juice in a bowl. Season with salt and oil and mix well.
Step 4 – To serve, detach the mussels from their shells with a knife, taking care not to break the shell. Place the green sauce in the other shell.
Step 5 – Optional: Serve the mussel broth in small cups with drops of lemon.