Put the chorizo in a large heavy pot with a lid over medium-high heat.
Cook until fat is rendered and meat is browned, about 10 minutes.
Remove the meat with a slotted spoon and reserve.
Add the onions to the pot and cook until softened, 6-8 minutes.
Add the chorizo back to the pan, pour in the wine and bring to a boil.
Drop in the mussels, cover and cook until mussels have opened, 4-5 minutes.
Remove the lid and discard any mussels that have not opened.
Season with pepper, add the parsley and stir very well until mixed and mussels are coated in wine and onions.