Melt the butter in a large saucepan, add the onions. Do not let them color. Add a tablespoon parsley to the onions. Add the wine and bring to a boil. Take off the heat and allow the flavors to develop for 10 minutes. Add the mussels and turn them in the sauce. Add some drops of lemon juice. 10 minutes before serving, turn the heat up high and cook , stirring, until the mussels open and detach themselves from their shells. If the mussels are cooked too long in advance they will shrivel. If they have to wait, cover them and take them off the heat.
