Step 1 - Wash and clean the mussels. In a pot, add the white wine and chopped onion. Cook over medium heat for 5 minutes or until the mussels open. Set aside.
Step 2 - Chop the remaining onions into small cubes, put in a bowl and add the sugar. Squeeze with your hands and wash with plenty of cold water. Put in a colander and drain to remove the water.
Step 3 - Put the drained onion, chopped parsley and lemon juice in a bowl. Season with salt and, finally, add the oil. Stir well.
Step 4 - When serving, remove the mussels from their shells with the help of a knife so as not to break them and put the green sauce on the back of the shell.
Step 5 - Optional: the choro broth can be served separately in small cups with drops of lemon juice.