You'll want to take your mussels out of their shells for this to make the soup easier to eat. It's a two-step process: Steam the mussels out of their shells and remove the meats, then strain and use the mussel broth for the soup.
Pour about 1 cup of salty water -- ideally boiled seawater -- into a large, wide pot with a cover and bring to a boil. Add the mussels and steam until they all open, about 3-5 minutes. NOTE: If you are using wild mussels, you will need to scrub and debeard them all first.
When the mussels are open, pick out the meats and put in a bowl. Toss with a little olive oil and set aside. Strain the mussel broth through cheesecloth or a paper towel into another bowl.
In a soup pot, get the olive oil hot over medium-high heat. Add the shallot and saute for 2 minutes, stirring often. Add the garlic and saute another 1-2 minutes.
Pour in the mussel broth, tomatoes, cayenne and parsley and mix well. Simmer uncovered for 20 minutes.
If you are adding pasta, pour it in. Cook gently until the pasta is done. Add the mussels and cook for another 3 minutes.
Serve hot with a nice medium-bodied Italian red, a dry rose or an amber beer. If you're not adding pasta, be sure to have lots of crusty bread.