Step 1Place the mussels in a large pan. Cover it and cook them over high heat until they open, 2 to 3 minutes, shaking the pan. Remove the meat from the shells and discard the shells. Strain the liquid that remains at the bottom of the pan through a cheesecloth and reduce it over high heat if necessary. You will need about 1 cup liquid.
Step 2Put the oil and garlic in a skillet and heat over medium heat. Add the tomatoes (if you are using canned tomatoes, discard the liquid first), and boil, uncovered, over medium heat for 5 minutes. Add 1/2 cup of the mussel juice and adjust the seasoning.
Step 3Add the remaining mussel juice to rapidly boiling salted water and cook the spaghetti until tender, about 9 minutes.
Step 4Add the mussels to the tomato sauce, reduce the heat to low and heat them gently for 5 minutes. Stir in the parsley.
Step 5Drain the pasta and turn it into a hot serving dish. Pour the sauce over it and mix well.