In a large suacepan or Dutch oven, cook the onions in the butter over medium heat until soft, about 5 minutes. Add the garlic and chorizo to the onions and cook until the sausage loses its raw color, about 5 minutes. Add the harissa and red pepper flakes, and then turn up the heat to high.
Once fragrant, degalze the pan with the wine, allowing to bubble for a minute. Lower the heat to medium, add the chicken stock and bring to a simmer. Add the prepared mussels and cover to steam until mussels open, 3 to 4 minutes. Add the cream and stir. Serve immediately.