Melt butter in a large, heavy stockpot over medium heat. Add onions and shallots; cook, stirring occasionally, until soft, about 4 minutes.
Add garlic and red-pepper flakes; cook, stirring occasionally, about 2 minutes, taking care not to burn the garlic.
Add wine to the pot and bring the liquid to a boil.
Add tomatoes and chorizo. Reduce heat to medium-low and simmer about 10 minutes, stirring occasionally.
Season with salt and fresh cracked pepper.
Add cleaned mussels. Cover and continue to cook, shaking the pot occasionally, until mussels open, about 8 - 10 minutes. (discard any unopened mussels)
Add basil; toss. Serve immediately with crunchy french bread!