In a large saucepan 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, deglaze 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 prepped mussels and cover to steam until the mussels open, 3 to 4 minutes. Add the cream and stir. Serve immediately with plenty of sliced French bread for dipping.
Cook's Note: If harissa cannot be found, substitute 1 tablespoon of tomato paste plus 1/4 teaspoon cayenne pepper.