To make sure your mussels are extra clean, before cooking, soak in water for 30 minutes, then remove the mussels and transfer them to another bowl by lifting them out of the water, so whatever sand or dirt stays in the bowl.
In a large saucepan cook the onions in butter and olive oil 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 cut tomatoes, cayenne and red pepper flakes, and then turn up the heat to high. Add white wine, stir. Allow to come to a slow boil (about 10 minutes), stir and reduce heat to low. Add the prepped mussels and cover to steam until the mussels open, 5 minutes. Serve immediately with plenty of sliced French bread for dipping. Sprinkle with chopped parsley.